I could live without shore power up here in the PNW, but I sure am glad I have it. (OTOH, this is on a CD25 which is more likely to have 120v AC needs.) Many reasons really, but here are 4 from me:
1. Easy to hook up a small generator and power the several outlets or hard wired appliances while at anchor. There can be many reason to need 120v AC in the cabin for short periods such as microwave, laptop chargers, small vacuum, etc.
2. If at a marina in wet, wet weather, I can easily plug in an electric heater to dry things out, or to supplement not quite adequate onboard heat.
3. When at anchor I like to charge my batteries via a high amp smart charger off the generator rather than via the engine alternator. AC wiring makes it a non-issue to run the charger whether at home, at a dock, or off the generator.....it's all the same hook up.
4. When working on the boat at home (on the trailer) I can plug in power tools to any in-cabin outlet instead of a extension cord that has to snake its way in.