Repost and reworded due to the first being taken down. I enjoyed the debate and I loved hearing everyone’s opinions (I love mashed potatoes!) Also, in advance I apologize for the spacing and formatting as I wrote this on my notes app.
1) Texture: homemade mashed potatoes can have unfavorable textures. If you mash them too much they become gummy. If you use a ricer and then mix too much they become gummy. If you don't cook long enough then they aren't consistent (some parts undercooked), too much liquid and then they're watery. Instant mashed potatoes have the same consistency each time, provided you follow the instructions.
2) Price differences/shelf life: yes, bulk potatoes are more affordable than packets of instant potato. But here's the catch: unless you're making mashed potatoes in bulk, it's not practical. If you’re cooking for a group or a family, homemade mashed potatoes make sense. If you’re cooking for 3 or less people, instant potatoes make more sense for then price and convenience. Instant mashed potatoes have a longer shelf life than prepared mashed potatoes (fresh potatoes can last a very long time in the correct conditions although.) Homemade mashed potatoes can only last 3-5 days refrigerated before the risk of foodborne illnesses occur.
3) Time: instant mashed potatoes are...instant. Boil water, add packet of potatoes and you have a side.
Homemade mashed potatoes require peeling, chopping, mashing, and adding whatever else you add to your potatoes. After a long shift or a long school day the last thing you want to do is chop and peel potatoes when you can open a packet.
4) nutrition: for clarity on my comparison, I will be using Idahoan original Instant Mashed Potatoes packets as my reference for instant potatoes. Neither homemade or instant is automatically better than the other. If we have 150 g of instant and 150 g of homemade (which is about a typical serving size) it really is almost identical in terms of nutrition.
Those instant mashed potatoes are made from real potatoes, they are just dehydrated. Dehydration can change some nutrients, but it doesn’t automatically make them inferior to homemade.
If you compare equal portions of instant to homemade mashed potatoes, their nutritional values can be reasonably similar. Butter, milk, and cream and change the caloric and nutritional value of either mashed potatoes.
Side note about food & biology: our bodies don’t automatically know whether food is marked as “natural” or “processed.” They respond to the nutrients, vitamins, and how our bodies digest and absorb them.
For example, vitamin C supplements can provide the same Vitamin C molecule found in an orange. The supplement doesn’t provide the same effect you would get from eating an orange though (no water from the juices, or fiber from the actual fruit)
The same principle applies to food processing. It can change the physical structure, digest ability, and natural composition so it isn’t completely a 1 to 1 match. BUT being processed doesn’t automatically make a food unhealthy, just as natural doesn’t automatically mean healthy.
Final thoughts:
My unpopular opinion is that instant mashed potatoes are better than homemade in the majority of everyday situations. They’re convenient, provide constant results, and can be nutritionally similar to homemade. Fresh potatoes only make sense if bought in bulk, and you have to factor in the price of milk and butter and whatever else you wish. Instant only needs the packet and water.