animal go outside

[Rei]

what's the benefit of letting animal staying outside? and when not to let them stay outside?
do i still need to feed them if they stay outside?

does their heart grow slower?

[Piperbird]

If you mean your ranch animals, letting them outside when you have grass means you don't have to feed them. The pig and luwak need to be outside to find truffles and beans.

Don't let them out in rain or snow.

As long as you pet them, and make sure they eat (inside or outside) their hearts will still raise.