Well lets say you work at TechCompany. TechCompany makes most of its revenue off of ads, but it has a bunch of side bets in classifieds, AI, self driving cars (or so its rumored), virtual reality, chat, even some finance tools. Now you want to go to NewTechCompany to go work on something there. But $TechCompany says you can't do that, they are a competitor, you can't work there for 12 months like your noncompete says! And you reply well no, I worked on search engine stuff here, I will be working on big data tools there, its completely different! And TechCompany just says- "No, they are also a tech company. You can't work there. On anything."
So you think about this for a minute, and go back out and go get an offer from an ad company (in the digital space). And again TechCompany comes back and says "No you cant work there, they are a competitor! We do ads too!" And you come back and are a little pissed now, and say "I have never worked on ads in your company before, I have no idea what that is all about, I don't have access to any of that stuff! I am going and stuff it!"
TechCompany now comes back and says "well we have an army of lawyers, we have already sent a letter to their legal department saying we intend to sue." This ad company is small and does business with TechCompany, and doesn't want to upset them or get into a long and expensive legal battle. They rescind your offer...
You now look at what else is out there... healthcare? probably not... they have some forays into that too. Insurance? ok maybe. A consulting firm? The options are just... small.
This by the way is not some completely fictional scenario. These agreements are meant to intimidate you and make it more difficult for you to leave. I had an egregious noncompete put on me, that essentially said I could not work in any business the company was involved in (which was ever expanding- though in the financial space), and at first they wanted it to be a fixed payment for not working, which while the number was equivalent to about 6 months of my base salary (which was only half of my total compensation roughly), I knew people at that firm that had been there 15 years. That number they signed was now paltry. They relented and said it could be based on base salary for that year, but that was it. I swallowed the bullet because I liked the company otherwise and the pay was great. Later I renegotiated my comp toward my base salary to make sure I was taken care of if I left.
Anyway, the specific details in my situation don't really matter. My wife has also been bent over by a non-compete in a similar fashion. You need to read this thing thinking about the worst case possible scenario if the company wants to bend you over, and nothing less. Taking these lightly puts you at risk of a world of hurt. My wife had to step back and take shitty jobs for 2 years because of a bad noncompete (for a place that she worked at for 4 months before she was let go after a re-org) and her compensation pretty much halved during that time- we talked to a lawyer, he said yeah you will probably win in the end, but it might take a year, a lot of money, the outcome is uncertain, and no one is going to want to touch you while the case is ongoing. DO NOT take these lightly.