I have an Insider's membership, though I'm not as frequent a user as others on this forum - hopefully, one of them will be able to correct me or add where I go wrong!
Basically, you use your membership ID # to keep your preferences and info on file, like National. Periodically, you get special rental codes and coupons just for Insider members. Free additional valid driver (my 22 y-o DD is underage, added costs).
You can use the kiosk or online check in to bypass the counter, though I think most customers now get that perk if they fill in the "Save Time" form connected to your reservation.
I usually check in at the kiosk with my res. # or cc, it prints out my forms; walk out to the
Alamo section, head to the aisle with my currently booked category and choose a car. Drive to the exit, where the attendant asks for my forms and DL - drive away!
To check back in, drive into garage, attendant will show you where to park, check out your forms and you head to the terminal.
HTH!