The FB pixel is how FB gets to know about conversions. If your company runs FB adds, FB encourages you to let them know about which users converted to paying customers so it can improve the targeting of your ad campaign. And if you offer a free trial and the upgrade to paying customer happens inside the user dashboard, presto-bingo Marketing now needs you to install the FB pixel there.

It's disappointing that this is what the internet has become, and I'm happy to see issues like this brought forward. I can see value in feeding back conversion events to an ad network, but the "just let us run code on your page" style of integration needs to stop. Give developers some API to explicitly send such an event, if they really need to.