I guess it depends on what you want to get out of LinkedIn, and your situation. Personally, my relationship with colleagues/boss is such that if I think about leaving they'll know about it from me first, and if I added a recruiter as a contact they wouldn't even think about it. But apart from that, my use-case for LinkedIn is primarily connecting with people I do/have work/ed with, to help us stay connected should we need to work again in the future, and so other people can see who I know and who knows me. If and when I leave my job, I won't be looking at adverts or to be contacted by recruiters, I'll be leaving for a job offered by someone I already know in the industry.

But appreciate that for other people, they might want to use it completely differently.

It provides a sense of validity to people who may know your boss and see you, as an employee of theirs, as being competent and proficient at your job role.

Say I work as a design engineer. My boss is probably older and more experienced. He may have worked at competing firms previously and has connections there. I connect with him, and he's connected with former colleagues. They want to recruit and so start looking around the networks of people in the industry and region.

Because I'm connected to my boss, I am more exposed in their ads. They see the mutual connection and consider that partial confirmation that if I'm employable under him, I'm probably a decent employee.

Networking with existing colleagues, including your boss, helps create exposure of your profile to others in the industry by utilising the connections that your (usually more experienced and more connected) boss has. I'd almost be willing to bet that connecting 'upwards' to more experienced people is on the whole far better for your profile exposure than connecting 'downwards' to lesser-experienced employees (e.g.: you as a mid-level employee connecting to new graduates entering your workforce).