The 501st legion, which is the biggest of these groups, was founded in 1997 so it predates the release of the prequels. If you’re into building things, the stormtrooper armor is a lot more interesting than most of what is a available on the rebel side.

The 501st is all about Star Wars fans who like making costumes working to get others into Star Wars and giving back to the community.

I’m going to directly quote Wikipedia here:

Giving back to the community is one of the 501st Legion’s highest priorities. Because of this, the 501st Legion proudly refers to themselves as the “Bad Guys Doing Good”. Members regularly participate in events to raise awareness for charitable causes, from walk-a-thons to blood drives, and provide opportunities for fundraising through events such as their “Blast-A-Trooper” game, where donations are collected from the public to target armored characters with Nerf blasters.

In September 2016, the 501st Legion announced their commitment to a first-of-its kind international endowment in a unique partnership with Make-A-Wish. The 501st Legion Make-A-Wish Endowment Fund will allow Make-A-Wish America and Make-A-Wish International to grant more Star Wars-related wishes to children diagnosed with life-threatening medical conditions. The endowment fund allows Make-A-Wish to grant wishes, alternating between the United States and international locations every other year – a first for an endowment of this kind. The Endowment relies on contributions from its members, as well as the public. With a goal of $150,000 in the fund by 2021, the Endowment fund will grant wishes from the interest accrued; as the fund continues to grow, so will the number of wishes granted by the endowment.

The 501st Legion never charges a fee for an appearance, but they do welcome donations to a charity in honor of the Legion or the local Legion unit. If an event host does not have a charity of choice, Legion members frequently direct the donations to the Endowment Fund. In cases where the event host is itself a charitable organization, a donation is usually not accepted by the 501st Legion as they volunteer their time for that charitable organization.

In 2016, the members of the 501st donated over 182,000 hours of community service, raising over $889,000 USD in direct donations, and participated in events that helped raise over $46 million for charities worldwide