I too have gained an investor, multiple clients, various job inquiries, and ‘hellos’ and other messages from acquaintances and friends both recent and from the past.
Employers are often more comfortable with job candidates if they see a professional LinkedIn profile and robust professional contacts.
Actually one advantage that you just mentioned here is how one gets to see who you might be linked in to who you may have in common, that you would never otherwise know you had in common.
If there is someone I really want to get to know, I'll make it a point get their email address to introduce myself. It's a "cold call" for sure, but when the lead in intro starts "Former FDA Review Chemist ...," like mine is in my line of work it instantly gets their attention. I usually make a contact.
Not impossible to build a respectable rolodex. And what's more, you can download your contacts directly into a spread sheet and sort them as you like.
Linked-in is an invaluable tool for my business.
FReegards!