Dialation may not be necessary every single time for a healthy adult, but it’s good to do occasionally as there are serious diseases that can only realistically be checked by getting a good look at the back of the eye.
Puff test can be vision-saving. It takes almost no effort, and is an excellent screen for glaucoma. Glaucoma is irreversible, and catching it early is essential to saving your vision. It’s not as accurate as other measures, but it’s fast (they’re usually doing other tests simultaneously) and easy so why not do it? But if you want the extra accuracy you can ask for the applanation - any decent optometrist will do it for you if you want. Some people prefer it.