Is being a maths "tutor" volunteer at sites like MATHNERDS useful in terms of "good impression" for future academic prospectives?
If you apply for grants where "Broader Impact" (or something like it) is a criteria, then a demonstrable history of volunteer and outreach activities related to your academic field of study can be helpful. Same goes for awards that consider "service" as a criteria.
(The reviewers of my NSF graduate research fellowship application commented favorably on my volunteer experience, which I mentioned towards "Broader Impact".)
This will likely depend on your discipline and the type of volunteer work that you engage in. It can't hurt, as at the very least it shows your're interested enough in the field that your're engaging in it informally. As for how much it will benefit your prospects, that will depend on the type and extent of volunteer work and how it fits with your prospective discipline. Establishing a consistent history of volunteering with a particular site will demonstrate more commitment than volunteering at many different sites for briefer periods of time. Assisting with moderating or engaging with the management of the site's services (if an option) also demonstrates a higher level of engagement and responsibility, beyond a casual hobby or side interest.
As for whether potential academic programs or jobs will look favorably on your volunteer work will depend in part on their interests and areas of emphasis. A program which emphasizes the application of mathematics in industry may not give volunteer tutoring much weight in the applications process when compared to a student who has had "volunteer" internships in industry. In contrast, a graduate program or professor that emphasizes the use of web technology to enhance math education would probably view your experience more favorably because it aligns with their mission.
In my own experience in behavioral sciences I completed several lab-based internships in biology and psychology, which emphasized scientific method and theory, including proper record keeping, sterile technique, and data collection. This was viewed favorably by the more research-oriented graduate programs I applied to; other applicants had mostly volunteer experience with clinical mental health populations, which those programs did not give as much weight. However, at one of the program's where I was wait-listed, a faculty member informed me that they were concerned I had no experience (even volunteer experience) working with individuals with serious mental illness, and it counted as a strike against me. My research experience wasn't seen as negative, but it didn't carry as much weight in the eyes of that particular program.
In sum, demonstrating a consistent track record with a volunteer organization will demonstrate commitment, interest in the field, and maturity. However, the amount of benefit you will get in terms of future academic prospects will likely depend on how well that particular volunteer experience matches with a given program's academic interests.