1. Sebastian Joseph-Day has been successful in Brandon Staley's defense before
A trait of Brandon Staley thus far in his head coaching career is that he likes 'his guys'. The Chargers often sign/claim players off waivers who have either worked with Staley before, or have played in a system very similar to his (ie. Fangio's defense with the Broncos). Some examples of guys the Chargers have claimed mid-season who fit this mold include Eric Banks, Trey Marshall, and Essang Bassey.
Los Angeles Rams defensive tackle Sebastian Joseph-Day would certainly fit into the category as one of Staley's guys. Despite having an under-the-radar 2019 season, Joseph-Day really broke onto the scene and established himself as a premium player during the 2020 season. Not so coincidentally, this was the same year Brandon Staley was the defensive coordinator with the Rams.
Joseph-Day established himself as a versatile interior defensive linemen and flourished alongside Michael Brockers and Aaron Donald with Staley at the helm. During that 2020 year under Staley, he registered three passes batted down, one forced fumble, one sack, 35 solo tackles, one TFL, and four QB hits, all whilst only playing on 40% of the team's defensive snaps. Joseph-Day also tallied 30 run stops good for 3rd in the league, but led the NFL in run stops per run defense snaps (0.144).
Having already established himself as an elite DT in Staley's defense, signing Joseph-Day in free agency to help solve the issues upfront fits right in with the Chargers' needs.