Here’s the thread for this proxy: Roblox Discord Webhook Proxy Server
And here are statements from Discord and lolpython (lead discord backend dev): x.com
Edit: As for the second question, I don’t know. One reason would be accessibility since Discord is free and most people already have an acct. Another is collaboration since you can invite your team members to your channel. Other services require you to pay to allow other team members view your logs (if you use discord as a logging service).