It's a good partitioning scheme for Cpanel servers.
You can also put your /var, /usr and /home under one '/' partition, but it's not a 'must'.
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They got 2 nice warnings why you shouldn't have /usr and /var on different partitions.
/tmp can be a file or ram (tmpfs) so you can grow it / shrink it.
In /var mysql hosts DBs, if you decide to make it a different partition, make it big. For 300 WPs it might need more space for DBs. I got a server with almost 150 accounts and it's already at 50GB. (But that depends of course).
From the other hand having multiple partitions helps with IO problems (you could possibly find out quicker where the problem is located with commands like iostat. You can understand if for example some heavy write or read in /var it's mysql or /home, a user or /usr apache, etc).