In American English at least, we would usually just say "practical" not "practical in use." "Practical in use" is a bit redundant.

Yes, I agree with what John said above. just saying: "It's practical" is spoken English. Saying: "Practical in use" is something more often seen in written English. Like product description or something like that.

Yes, as long as the reason for it being "practical" is justified. For example: "This rain jacket is very practical in use, because it not only protects you from the heaviest of rain falls, but it also protects you from strong winds. Does that help?