what are reason why we don't want to change resources on machine which are managed by vRA?
Don't understand the question, can you explain?
Yes,
What is reason that we don't want to change resourses of vm manged by vRA within vCenter?
Such as increasing disk size / ram/ cpu within vcenter ?
Because vRA maintains the state of its deployments internally, and when you change those resources yourself you throw it out of sync. So you should do the resizes through vRA if you can.
