In general it’s helpful to fill them in.
With some taxa and life stages, etc it is particularly important as they get a bit lost within the system otherwise. For example, a moth larvae expert may use a filter to just identify larvae observations not adults… but if the observations do not have the life stage annotation, the expert will not be able to find them.
No, it’s great to add annotations to others to help!
As an observer it’s fine to leave them blank also though.
Not everyone wants to go into detail on every observation they upload.
Some may come back to add detail later.
Some may not be able to add the details easily on their phones.
Some may not know the correct sex, life stage or host plant, etc.
There are many valid reasons for leaving it blank.
Surely add them where you’re certain! Observers don’t add them because it takes time, can’t be done on upload page and sometimes they just don’t know! Also, some annotations are available only for low-level taxa, so e.g. if they upload an Animalia, there’s no way to annotate it as larva.