Everyone has given OP great info but I'd like to throw out one more thing from recent experience. Hubby is GM and until two months ago always booked our reservations together. Due to some glitch with DL credit, we had to book two separate reservations and then have them linked.
It didn't go as smoothly as you would think. Hubby has SKY boarding and I had lower boarding status but that was taken care of by understanding GAs. Hubby got TSA precheck and of course I wasn't granted that benefit. We used carryons only so bag fees weren't an issue. Out of 3 flights, 2 had equipment changes and our seats together were rows apart with me in a middle seat. I understand that sometimes things happen but for that trip, linking the reservations together didn't work for us!