Hopefully you haven't noticed this, but there has been a marked drop in misspellings and missing words in my last few posts. It is definitely not the case that I have suddenly gotten better at the English language, but rather the fact that I've been working with a freelance editor. While it's great to have posts polished before I post them, this isn't really the main advantage to me.
What makes it worth while is the extra pressure to get these posts written. Maneesh Sethi talked about this idea when he hired someone from Craigslist to sit by him while he worked, and slap him if he slacked off. He argues that "if you add a social element to the work that you do, you will become more productive," and I've found that to be absolutely true. You'll feel much more pressure to not let someone else down, no matter how casual the expectation is. When I get an email "nagging" me about a new post to edit, I feel guilty if I don't have one ready.
This works the same way with fitness. When you work with a personal trainer, it's great to have someone to tell you what exercises to perform. The main advantage, though, is the added accountability. If you are paying someone to help you work out, it's a bad feeling to tell them you stayed in and ate ice cream instead of going running like you said you would.
So, if you are serious about wanting to improve some aspect of yourself, consider finding an accountabilibuddy to keep you on track. Even if it costs you a little money, it is absolutely a worthwhile investment.