@deviantollam Eugene (@Gargon) runs 2 servers mastodon(dot)online and mastodon(dot)social, and he is the main coder on the Github branch. So the donations from Chris's post will go to help support the servers and Eugene's time in improving/maintaining the code/features of Mastodon.
Each instance will have it's own costs associated to it's bandwidth/space/hosting/etc. There are other Open Source projects on the Fediverse that also accept donations, some for alternative backends (Pleroma, Freindica, HubZilla, etc) and some are for alternative front ends/applications (Glitch-SOC, Pinafore, Sengi, Toot!, Soapbox, etc.)
There will be load on each of these servers as their Local and Federated timelines continue to grow. And Chris's biggest point is to make sure people are aware that there isn't any Advertisers or Data Logging/Selling going on to facilitate these costs. Many of us would say donate where you wish, but to always try to support your local instance's admin since they help your experience more immediately.