I buy on Bandcamp because it's significantly less expensive.

Artists set their own prices and many are below the $0.99 price point.

A lot of albums are "name your price" and you can't beat that.

In Bandcamp you can buy with Paypal. I appreciate that. On the other hand Bandcamp is a bit more like that brick-and-mortar record shop next door compared to those huge corporations. Music and corporations don't match too well for me. I know my reasoning is out there for the most, but can't help it :)

to be fair, you can listen to the whole song on Bandcamp before buying compared to 90 seconds on iTunes

If I remember correctly, Bandcamp takes a 15% of revenue (it drops to 10% once you go above 5K) while Apple takes 30%.

Better audio quality, cheaper price, better shopping experience, can buy physical products eg vinyl and tshirts, shopping page is customized by artist and has links to their online presence, bigger cut goes to artist, some tunes given away free, traffic is driven by music review blogs, the list goes on and on.

Did I mention that I love bandcamp?

Because there are many people who simply don't. They may require lossless audio or local storage instead of streaming. I know I do - and none of those services offers these options. That said, bandcamp, boomkat or qobuz are indispensable lossless audio stores for me.