I have been to several trans social events, and enjoyed them. Typically dinner at a restaurant, with 8-10 people, including some SOs. The last one, we had about six trans women, one trans man, and three spouses.
I find that I am more outgoing since I started full-time. Well, okay, "outgoing" may be the wrong word. Less introverted would be more accurate. I find myself longing for social interaction, and these events are a good excuse to get some.
Also, there are some people in that group who are not as far along as me, and some whose home situation doesn't allow them to express themselves. Social events are a way of supporting them and providing an opportunity to present as their true selves. For people who are uncomfortable in support groups, it lets them meet with other trans folks, helping them to normalize their situation.
And speaking of normalization, a table full of trans folks in a busy restaurant doesn't necessarily attract attention, but we are visible. I think visibility is important, so that cis folks can see that we aren't freaks who hang out in the creepy parts of town. They see that we are just normal folks out for a meal together, like anyone else.