There is no need to undercoat every cake. But there are many than benefit from an undercoat. Any cake that has a lot of crumbs on the outside edges has to have un undercoat so the crumbs are trapped in the undercoat and do not show through on the finished cake.
Typically, carrot cakes, banana cakes and others finished with a powdered sugar based frosting do not need an undercoat because the cakes do not crumb much and also because these types of frostings can be put on thick enough one step. However, if you have any question, always apply an undercoat.
While it is an extra step to undercoat, sometimes referred to as a crumb coat, the finished cake will be so much better looking. This is particularly true for cakes finished with Italian or French buttercreams or a ganache. Ganache will follow the side of the cake very closely making for a rippled effect on the sides.
Baking gets better when you subscribe to receive this PDF on getting the most out of baking including information on equipment, ingredients and baking equipment.
The undercoat is applied with the same finish as the overcoat but doesn't take nearly as much ganache or buttercream.
Prepare your ganache. 
It will most likely be of a runny consistency and need to be thickened up before applying to the cake. Pour some of it out onto a rimmed baking sheet and refrigerate it briefly to a spreading consistency.
Cover the sides of the cake first. You do not have to take a great deal of time here, just get them covered. 
Plop a blob of ganache in the center of the top layer and
with an offset spatula, move it out to cover the top evenly,
allowing it to overhang the edges.
Line a bench scraper up to the side of the cake and go around the cake completely. A turntable is really helpful here, but if you don't have one just go around as far as you can, turn the cake and continue. 
The excess ganache or buttercream will now stick up over the top edge of the cake. To remove this and complete the undercoat, use a large offset spatula and, starting at the far edge of the cake, remove the ganache by moving the spatula in towards the center, gently gliding the spatula up and off the cake around the middle. Go around the entire cake in this manner.
You have now finished the undercoat except for two quick steps. 
Clean the bottom of the board with a paper towel wrapped around your finger. 
Once the bottom has been cleaned, slip a flat metal spatula under the cake, to free the ganache so it won't pull off when you move it for the final coat.
In two weeks, I will be blogging this Chocolate Strawberry Ruffle Cake in which the cake will receive an undercoat before finishing it.
Eva says
Ms. Fletcher, I agree with Vicki. I luv luv your blog. I am a 61 year young self taught home baker. I have learned a lot over the years, but have learned even more since finding your blog. Thank you. I can't wait to get the details on the chocolate cake above. I have struggled with choc curls (the big ones like above or big tbsp size). I could never figure out how bakeries make them. Looking forward to all of your posts.
hfletcher says
Hi Eva: Thank you for the kind words. I will let you on a secret. Most bakeries buy small, medium, large and teeny-weeny chocolate curls. Bakeries can buy almost anything. I didn't like the taste of the chocolate decorations because they used a stabilizer in the chocolate to help them hold their shape. So, of course, we made our own. No wonder I worked 14 to 16 hours a day for years to get started.They do take practice, mainly because you have to get used to judging the temperature of the chocolate -once you get that, it's easy.
Vicki says
I am so impressed that you are willing to share your wealth of knowledge with all of us! I read and enjoy every post! I am just an amateur, but all of your info resonates with me. Your blog has such useful information without all the "fluff" that most blogs seem to include. THANK YOU!
hfletcher says
Thanks Vicki for taking the time to send this comment. I have never believed there are "secrets" in anything I do. If I can help anyone, I am happy to do it.
hfletcher says
Hi Ayoma: Thank you for your kind words and letting me know you appreciate the blogs. It means a lot.
Manisha says
Hi Helen
Chocolate strawberry ruffle cake is so beautiful!!! Looking forward to the post on it.
Undercoat really helps. Specially with buttercream cakes, I feel it gives a better finish and why not as it is not too much work.
Just wanted to confirm if you have been getting my emails?
Love
hfletcher says
Hi Manisha - I have gotten them. Just overwhelmed this time of year wth graduations, weddings, engagements, etc. at the restaurant. It sounds like the new kitchen is wonderful to work in. Mine at home is small but I like everything close to me.
I can't quite figure out where the time is going. But it is sure going fast. Love to you.