You get BG2 (boarding group2) as star alliance silver, which isn't something that the alliance guarantees but united is just being nice.
https://www.united.com/en/us/fly/tra...g-process.html
(This BG2 is listed in UA benefits. I guess it implicitly gives you carryon bag for domestic BE but don't quote me on that, I haven't seen it in person. No premier access.
Any other star alliance silver benefits is more-or-less useless. You do not get the United silver benefits (e+ at check-in, 70lb bag instead of 50lb, free bag)