Grand Theft Auto Online has been available for almost 6 years now and it has received a steady stream of both new content and new players ever since. GTA 5 still tops sales charts, and Rockstar is still releasing new major content — such as the upcoming Casino DLC.
But if you're one of the countless new players jumping into Online each day, all that content backlog might seem too daunting to care about. That said, you still should.
Each subsequent DLC has typically been bigger, flashier and more lucrative than the last. Getting enough money for the best gear, vehicles and properties is a tough task, so it would make sense for players to grind the highest-paying activity and it alone.
However, in many cases, major updates build upon the foundations of those that came before. There are plenty of benefits to going back and trying out past GTA Online content, and if those wouldn't be enough, Rockstar is giving some incentive to do so as well each week.
Bonuses
This brings us to the first and most obvious advantage of catching up on past DLC. Rockstar's weekly bonuses in GTA Online often increase the payouts for missions related to previous updates, such as CEO work, Special Cargo Deliveries, Bikers Businesses and more. If you already have the necessary properties — most business-related content requires the ownership of a specific facility — then there won't be an initial investment to worry about.

While the baseline earnings are usually always the highest in whatever the latest major DLC is — we don't count racing themed updates in the vein of Stunt Races and such — the 2x rewards bonuses can make legacy content like Import/Export an absolute cash cow.
Experience
When speaking of experience, we don't mean the RP you accumulate with gameplay, but actual experience with playing the game. While the basic structure of major updates has been the same since Further Adventures In Finance And Felony — acquire supplies, produce the goods, deliver them in a sell mission — the particulars are mixed up in each case.
And then you have those updates which take a different approach altogether.
Everyone really ought to still be playing heists even though they're really old in GTA Online terms, as well as the new take on that structure provided by The Doomsday Heist. Bikers again offers variety with contracts, and you can experience the peak of pre-business GTA Online with Executives And Other Criminals.