While most are not legally binding, there may still be consequences for not complying with the agreed upon portions of the letter. A Letter of Intent is often seen like purchase or crowd-sourced orders, so they are often used to help secure funding or prove the value of the business. If the other party drops out or doesn't honor "in good faith" the letter, actual financial damage may be incurred to the company. In court, most often the judge will make decisions based on what implied intent is expressed in the letter. Sometimes there can be recourse if the parties do not follow through towards an actual legal agreement.
Without job-level specifics, it can be tempting to get stuck in an "I'm awesome" loop without giving enough specifics. If you have a general idea of what job groove you'd be seeking at this particular company, build your letter body around that. If you're truly just trying to get a foot in the door at a company, you can use clues from your research about the company. What does their website say about their mission and priorities? What do current and past employees say about the company in online chatter? Even without specifics, you can cobble together a pretty good idea of what the company is seeking in potential employees. Sell your skills and experience points that are special to you, and especially relevant to your industry.