5 years ago...
Common knowledge was that mastering was just a matter of loudness and the goal was to be louder than the guy next to you, but then something changed and today, common knowledge has turned back into dynamics instead of loudness, the problem is that self-produced musicians and recording studios don't have enough tools to make a record louder and yet dynamic.
Tools R Us
Many of those self-produced musicians and semi-pro recording studios are realizing that mastering is way more than just turning up the makeup knob in the compressor or putting a brick-wall limiter in the master busand they are considering options outside their own studios. This is a huge breakthrough, cause looking for help in the DIY era is really something.
Automated online services
There is nothing wrong with those services like LANDR but before you make a decision, you have to be aware of a couple of things:
1. The master is automated, which means that it will be ready in 2 minutes. The question is: For a record that took you months to get done, do you really want mastering that takes only minutes? Is that worth it to your music?
2. An automated service where you are the judge is kind of against the whole point of mastering, but don't take it the wrong way, in the end, you have the last word. The "being the judge" thing means that how can you take your record to the next level if you don't have an objective perspective? If you will pay for it anyway, why don't let someone specialized do the job and ask for changes if something is not satisfying you?
Here's a short video with 3 examples of my masters done for the same price you'll pay with these automated online services