Clients can give you permission in order to help them with their projects, like: job posting, interviewing freelancers, reviewing the work diary, ect.
If your not sure why your client has given you this permission its best to discuss it with him.
Oh yes my client asked me to manage his upwork account. but i wasn't sure about it that i can logged into my personal upwork freelancer account + client account at a time.
So i can browse my clients account also?