Why does ICMP Echo Request and Echo Reply packets contain a Data portion?

The ICMP Echo Request and Echo Reply packets contain a Data portion. For example this is an Echo Request packet (that was sent in Windows using the ping command):

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As you can see, there is a Data portion in the packet that consists of 32 bytes, and this Data doesn't seem to mean anything (it is just part of the alphabet).

Is the Data portion mandatory or is it optional?