A lot of selfhosted containers instructions contain volume mounts like:

docker run ...
  -v /etc/timezone:/etc/timezone:ro \
  -v /etc/localtime:/etc/localtime:ro \
  ...

but all the times I tried to skip those mounts everything seemed to work perfectly.

Are those mounts only necessary in specific cases?

PS:

Bonus question: other containers instructions say to define the TZ variable. Is that only needed when one wants a container to use a different timezone than the host?