Bitcoin fees are voluntary and not required. However, miners keep the fees for including transactions in blocks as payment for service. They can choose which transactions to include and, of course, are more likely to go for those that include a fee. Tx's with no fee will almost certainly be in a block eventually. Those with high fees are often prioritized. In the big picture the fee gravitates toward the lowest profitable amount.
When I sent my transaction I set the slider towards the middle because I wanted it to be quicker
Yes, the fee is not terribly low. My estimate for 18 blocks shows 12553 satoshi/kb currently though. This can happen as the information core uses to predict the fee is based on the past. The future might look different. We just have a lot of transactions waiting for a confirmation currently.
Note: I did a reboot 12/12
Edit: while we are chatting away, the TX is confirmed (4 currently).
Bad Gateway ;-)