I joined a unit for several reasons and I have been able to get a lot out of it. Let me count the ways
1) In any match, a coordinated group has a distinct edge. Especially with an experienced player giving a plan, calling targets, etc. Even if not everyone is super good if people follow orders it will work much better than everyone doing their own thing.
2) Related to the above, Community Warfare. This is where I was recruited. In CW, you just cannot win if you are not coordinated and you kind of need as much of a group as you can to drop. Granted the factions do have their own teamspeak and you can often pick up enough that way, but it's best if you have a unit who knows each other's builds, playstyle, strengths and weaknesses. Pugs seem not to be great at following orders or using voice comms, both of which are required for CW victory.
3) Mentorship and learning opportunities. Usually in a unit there are lots of people who will be more than willing to help you, give advice, etc. Mine has been great in this regard as they are very newbie friendly compassionate folks. They have helped me with builds and given great advice. Between that and their observations it helped me grow as a player much more than I would have on my own, even doing research as I tend to do.
4) Camaraderie. an extension of point 3, joining a unit is a great way to make new friends and have fun playing together.
I could go on but I am sure you get the gist. Units are great great great. The more friends you have the better. I suggest you fill out your friends list as much as you can whether you are in a unit or not because that will help you find groups and fill them out.