changes.rst: use and users of optional tags

Hi

in the table in changes.rst some tools are marked as optional, sphinx has a
foot-note and the paragraphs below have optional markings as well (they do
not fully match the ones in the table).

What is the optional tag for? as in what is basic use vs. what is extended
use of the kernel (requiring the optional software)?

Who uses the optional tag? I can imagine there used to be people with scarcely
memory, bandwidth or computing power. But I don't think this is still the case
nowadays. Who else would use the tags?

Thanks
 Manuel