Great question.... main reason is to ensure emails coming through TennisPoint.com get delivered to you. The "reply to" is your personal email and 90% of all email programs will obey the "reply to directive", but not all do.
FYI, that email address --> member+_031ee8a12ad5 is a combination of the text "member" and a unique encrypted number of your account on TennisPoint. Since the email comes from TennisPoint.com servers, this helps prevent spam false classifications but also helpful that we can automatically forward to your own email account when a reply comes to us versus to you directly.
The "longer answer" is all the big email players (bellsouth/yahoo, comcast, aol, gmail, hotmail, etc) enacted the DMARC anti-spam/spoofing measure about 2 years ago and we had to change things up then. A key DMARC rule is the "from email address" MUST match the sending email server --> TennisPoint.com. Previously we were able to use other "email sending mechanics" to get around this, but DMARC unfortunately removed that ability -- supposedly "phishing/spammers" were exploiting/hacking it and supposedly the "from email" is more secure to check against.
Here is more info on DMARC: https://dmarc.org/wiki/FAQ