My understanding of abstaining (from meat, other rich foods, or a pleasurable but not necessarily healthy food or practice) is to help you use the season of Lent to focus and reflect on where your life is, and where you'd like it to be. This is easier to do from a "place" or lifestyle that is simplified and freed of distractions and temptations.
I've also heard it explained that giving up something you enjoy helps keep you grateful for what you have.
Some people use it as a personal test (a la 40 Days and 40 Nights) of their self-discipline or resolve.
I think the decision to do so or not, and what you choose from which to abstain, is less important than the reason. If it is just to satisfy some church's "rule", why bother? They won't know if you accomplished it or not. If you plan to actually get or learn something from the experience, then it's worth it.
Also it wasn't til I left the RCC and became an Episcopalian that I heard of the idea that Sundays IN Lent are not OF Lent and thus you can have whatever it is you've given up. I heard that from a priest who chose (and thus forced his family to choose as well, as he was the cook) to give up meat for the whole time. The only problem was he made such a federal case out of it to anybody who would listen that it completely defeated the purpose:
16 ‘And whenever you fast, do not look dismal, like the hypocrites, for they disfigure their faces so as to show others that they are fasting. Truly I tell you, they have received their reward.