My first mod - actually my first PV - is one of Old Goat's bottomfeeders. I like having a PV I can carry around for a day and a half without any maintenance. The battery lasts me about 24 hours (I generally sleep at max 6 hours a night, vape the rest), as does the bottle. If you're not into feeders, or you just prefer dripping, carts, cartos, or what-have-you, a box mod with the 14500 or 18650 battery will last forever between charges. My 14500 is rated at 900 mah, while there are 18650's rated at 2400 or so. It's the goofiest thing, but Old Goat attaches a lanyard via a jewelry-style ring, which means I carry mine around my neck all day. It's fantastic for driving, gaming, cooking, or whatever else I may be doing and I can't go digging or set it down nearby.
So, for me, the main reasons for a mod are the battery life and the convenience. Oh, and Old Goat's prices are amazing - they're right around standard 510 kit prices. Regular box mod w/no feeder is even more budget-friendly.