DD and I did the same thing you are last year (already had a trip planned and decided to join the walk) but we went "all in" and did some fundraising too. I am a pediatric pharmacist so it was a no brainer for me to raise money for a children's hospital like CHOC.
As far as teams-some of the teams have additional incentives you can earn with donations or special t-shirts. That would really be the main reason I'm aware of. We joined the Annual Passholder team and based on donations earned several additional Passholder team specific incentive items (my DD ended up one of the top 25 fundraisers on the team!). The AP team also had a special T-Shirt you got instead of the general one. So I'd just look at the team pages and see if you think there's a benefit for you. If you don't really plan to fundraise beyond the basic donation to participate and don't feel a particular allegiance to any team there isn't really a need to join.
Some people who want to be near the front get there VERY early. We weren't super concerned about that and left the GCH around 6:20am. The stated start time was 6:15am. We were about halfway back in the esplanade and then once we started moving ended up near fire station where we could kind of hear the announcers from the start line. We stopped several more times as the mass of people made its way in. It was actually uncomfortably packed for the first part of the walk. If I did it again, I would probably arrive even a little later to be farther back. As the walk went through other "choke points" in the park such as Big Thunder Trail it again became uncomfortable and difficult to move. DD (who was 9 at the time) actually had a bit of a panic attack from the crush of people which had never happened with her before. Because she was so uncomfortable we ended up bailing on the walk halfway through (when we crossed the Esplanade to DCA). Don't get me wrong- people weren't rushing or rude or anything. The overall atmosphere was very fun and friendly, it was just a LOT of people so just be prepared. I used to do runDisney events when they still had them at DLR and this was very different- much more of just a big slow moving walk. Again, this isn't bad or anything but more so you know what to expect.
Edited to add- I would do it again. DD definitely would not.
Also- I don't know where you're currently staying but we got a KILLER rate at the GCH through the CHOC walk. I don't know if they're offering that again this year but worth checking out!