When writing an email asking for PhD positions/projects
Quite simply, if your email to me does not explain why you want to do a PhD in my group, I am probably not going to respond to your "cold" email, unless your CV is so outstanding (several papers, good grades, working with people I already know, etc.) that I can't help but take a closer look.

So you need to make sure that your email not only expresses your research interests, but also explains why your profile makes a good fit for the group to which you're applying.

Also note that if you're sending a "cold" email and the professor isn't in the process of advertising a position, you still may not get a response.