Of course, buying is optional.
But, once you start to better your equipment, it's almost inevitable to advance in tiers, and, for most, the free fun with Starters stops at Master tier, stepping back to the second-longest tees. To come close to contention, you'd have to buy decent clubs there, e.g. the full set of G25 (L59).
The other path is to idle along without buying, revert to Starter balls, and enjoy the views (plenty of splendid ones around). This may become annoying though.
Sporting some ambition, you'd have to spend a whole lotta time in the "free credits" dept., suffer a lot of "being scammed" feelings, or spend some bucks for your leisure. It's not too expensive, say 10-20 $ per month for a modest player, with one or two phases of 50-100 $ when upgrading equipment. A night in town with your sweetie may be more expensive...
Seriously, some investment will be necessary if you want some fun, since, in the FTP business, also PTW is included!