I do both. I make instant mashed all the time for DH and I for regular dinners (I think I will be making it tonight actually!) I tend only to make homemade on holidays or on a special Sunday dinner or something like that. We both like both different kinds.
For the same reason that some people used jarred spaghetti sauce, frozen and canned vegetables, pre sliced and shredded cheese, instant rice, boxed or frozen macaroni and cheese, pre baked and sliced bread and rolls..... convenience.
A sstated DH liked them so I make them only when we're having a quick meal. 5 minutes for instant verses 20 minutes for regular is a big deal for some people.
We rarely eat potatoes anymore, so instant for DH and myself (and they taste great!...much better than years ago). We don't keep potatoes around just for us.
Only home made for holidays and large gatherings.
Homemade only. They really don't take that long, and the boxed are nasty. Plus the boxed are full of sodium and hydrogenated oils and preservatives - yuck.
I personally don't make mashed potatoes but my husband makes the best homemade garlic mashed potatoes from scratch. They are to die for. I would never eat instant mashed potatoes.
99.9% of the time it is homemade. We just all prefer them homemade. I do occasionally make flavored instant mp's (butter & herb) for me & youngest DS. That is once in a blue moon, though.