I only wrote to professors to see if they were taking a student (if I didn't know) or if I wasn't sure if they were still pursuing an avenue of research that was in line with my interests. In doing that, you introduce yourself slightly; some might remember you, but I wouldn't count on it. I received interviews from both professors I corresponded with and those I didn't. Thus, treat it as a way to get information for yourself, not necessarily as something that's going to give you a leg up.
I would also recommend keeping it to about a paragraph (max two short ones). I can't tell you how many times graduate students, postdocs, and professors emphasized that potential POIs will be receiving hundreds of these emails and will not have time to go through all of them. [A couple of graduate students I knew beforehand who worked for POIs I was going to contact told me quite bluntly to keep it to a couple of sentences at most!] So: Keep it short, direct, and to the point.
Some professors will answer with one very short sentence ("Yes, I am.") others will respond with a couple of paragraphs of information (at times original, at times a stock answer) and some will ask you questions and e-mail back and forth. Heck, some won't respond at all. However, the only thing I felt this correlated with was how new the professor was and how busy they were. I only mention this so that if you don't get a response or if you get short ones in return, you don't take them as bad signs.
Good luck!
