Why are you interested in MY program?
The only way to answer this question cogently–and never forget that your audience is a faculty member who knows a heck of a lot about his/her program and institution and has a fairly keen sense for BS– is to do a lot of homework.
First, you have to delve deeply into what the program faculty teach and research, understanding that every program offers unique expertise, even those within the same academic field. Begin this by going over descriptions of the curriculum, then reading faculty profiles, and even pulling up and reading some of their published work. You might not fully understand much of it, but it will still make some sense. Developing a dialog with some program faculty by exchanging messages and phone calls to get your questions answered can be invaluable in this discovery phase.
Second, you have to take some time and come to some decisions about what you want to learn or what research you wish to pursue while in graduate school. At a fairly specific level. Trust me, something that you read in step 1 above will resonate with you, if you are not already at that point. You will know what you like when you see it. These don’t necessarily need to be perfectly formed plans, but they have to be a lot more specific than generic platitudes. Going through this process prepares you to convince me in your application why my program can best serve your interests. Year-in and -out, applicants who make a strong case that they are a good fit for my program are those who receive offers for interviews and even admission, whereas otherwise strong applicants can be weakened significantly by doing this job poorly.