You don't have a background with Chris Roberts games I take it
Are you familiar with Wing Commander? With WC, Origin, and Chris Roberts, create the WC universe. Privateer was a non linear space exploration/trading/shot-em-up game based in the WC universe.
When Roberst left Origin and started Digital Anvil, then was purchased by MS, MS had Warthog of the UK developing a space game, that was altered to become Starlancer.
Why? Freelancer was the big game DA was developing, and in so creating a new gaming universe. So the idea with StarLancer is to introduce players to the history of how the Freelancer universe came into being. So with StarLancer you are fighting in the war mentioned in the beginning of Freelancer, trying to help the Alliance keep from being exterminated. Just like all normal flight sims, i.e. the WC series, the X-Wing, Tie-Fighter types, etc., SL is a mission based, go from one mission to the next with no flexibility design.
So SL = WC in this new universe from DA. Freelancer = Privateer. Freelancer is the go anywhere, do whatever game for this universe. Allows for more replayability with this design
So do you need to play SL? No. Should you? Yes, if you'd like to experience what leads up to the sleeper ships needing to get the hell out. SL doesn't get you up to the sleepers, but it lays the groundwork for why you would know the alliance was in bad shape and would want to leave
regards,
RogueOne