what do you still need the phone for since they added standalone capabilities?

You need an iphone at least to register the watch to the account, although I don't think anything forces you to keep it after.

> I don't think anything forces you to keep it after

Many watchOS apps and features require an iPhone in order to be set up. For example: Oceanic+, Waterlamma, Overcast, various Apple Health features, or Sleep Cycle. That's a pretty wide variety from my own experience so I suspect it's quite common.

Until that stops being a thing, we'll need iOS to maintain our watches. In some cases it sort of makes sense because you're using apps that function sort of like a sidecar to iOS apps, not a standalone app. There are several where this isn't the case at all though, and it feels a lot like unnecessary tethering which limits our ability to use an otherwise very capable device on its own.

I think we'll get there; other manufacturers won't have the same incentive to keep watch apps tethered to phones, and I suspect that freedom will become increasingly appealing to Apple Watch users. I could be wrong, but I imagine eventually that limitation will seem too ridiculous to consumers. I don't expect to happen for quite a while still, though.

Can the Apple watch last all day if it’s using cellular for data and doesn’t have wifi or a phone to piggyback from?

The Apple Watch Ultra can quite easily (I listen to podcasts, make phone calls, respond to messages, monitor sleep, and always make it through a day and end up charging the next evening), but it's extremely expensive for something that isn't properly standalone.

The other watches seem to come close, but should be topped up pretty much daily (with my level of usage) from what I understand.

The battery life is horrible if you try to use it a bit more often like you would a phone

Unfortunately several apps and features (both Apple and 3rd party) require the phone for set up and configuration.