2. It gives the LA Chargers flexibility in the 2020 NFL Draft
Aside from shoring up the offensive line, which would be incredible for the team, acquiring Trent Williams would allow the LA Chargers to be more flexible in the 2020 NFL Draft as they are taking away one of their biggest needs with a fourth-round pick and King.
There are multiple routes the Chargers could go. Perhaps this means instead of taking the best available tackle in the first round the team takes the best available player, which would either be Isaiah Simmons or Jeff Okudah, both of which would replace (and arguably improve) from King on the depth chart.
Or, the team could select a quarterback with the sixth overall pick and then would have more freedom in the second round. This could mean scooping up Patrick Queen if he drops, getting an elite wide receiver to be the WR3 or getting a lockdown corner that can play out-wide.
Without trading for Williams, the Chargers would almost certainly have to take a tackle in the first two rounds in the draft. If they want any longer they are not going to get anyone who is going to be much better than Trey Pipkins at left tackle.
That could take away the possibility of the team even considering to take the best available in the first, whether it be Simmons or Okudah. Both are very talented prospects and would make a stout Chargers defense one of the best in the entire league.
Williams not only makes the team better with his talent but makes them better with the talent that he allows them to also bring in.