Its not a necessity. But its a significant bonus that happens every year with up to 50% of that suddenly makes this 26 million just over 13 million.
Sitting on over over 60 vehicles through multiple sales, discounts and other opportunities to get them easer is also problematic in itself and leads to misleading numbers too.
Well it is in a lot of respects. Utilizing many of the bonuses given to you is a core part of any free-to-play economy. Making the most of events, sales, boosters and modifier rewards is core to gaining lots of SL.
Some players sit on hundreds of millions just by playing casually. Naturally if your chasing progression much faster, you wont retain as much, but there are also a huge variety of ways you can earn SL, making the most of modifiers and bonuses that do not "force" you to play limited ranges or modes.