Like Patrick mentioned, to take part in the WRS (or other officially-sanctioned GTP events), you'll need a GTP_ PSN. However, there's still plenty of other community series that don't require this.
Having the tag means you understand and agree with the GTP Online Racing rules and regulations 👍. While it can be a pain to lose all of your progress, the Seasonals do make it a bit less painful to quickly rack up the level to buy nearly anything, and the necessary in-game funds. My personal trick? I just use my GTP_ login as my main GT5 save, though that's because I've had it since before GT5 released. You can always gift your GTP account the necessary car each week 👍