If youve played even a little ways into the 5.0 story(let alone up to 5.2) then you already have an idea of how important the tower is to the story being told in Shadowbringers. Two things(that i wont say because spoilers) that happen in 5.3 make it even more important that the raid series must already be done on your character in order to move forward. The others are right, the content isnt hard nor are the ques overly long. You should be able to do the quests, ques, and raids in under 2 hours. They really are just bigger dungeons, that your party happens to team up with other parties to tackle(which actually makes them easier instead of harder, as you have more people who can cover for something if others screw up). As a DPS, you can literally just focus on killing the trash/killing the bosses and completely ignore the people who arent in your group. Then once you finish those two hours you never need to do them again if you didnt like them.

Side note: Anyone wanting to work on the relic for the current expansion(lvl 80 resistance weapons) needs to do the alliance raids from the last one(lvl 70) because the story is a continuation. The Return to Ivalice(lvl 70) raids are a little harder then the Crystal Tower(lvl 50) ones were, but the devs nerfed them to make them easier when they became a pre-req. Clearly this is intended to be accessible, and it really is.