Use is used when it's still true now: The key is used to open the front door.
Use was used when it's finished: That entrance was used before the new wing opened.
Check the time reference: now or a general fact, is used; a finished past time, was used.
If the room still serves that purpose today, write The room is used for meetings. Your wording tells the reader this is the current, regular function, not a one-time event.
If you're stating a general fact that hasn't changed, Salt is used to melt ice on roads works. The present form signals the practice is ongoing and still true whenever someone reads it.
If the event is over and tied to a finished time, write The room was used yesterday for a workshop. Your wording reports something completed, with no claim it still applies.
If you need to contrast the two in one sentence, try The old kiln was used before the workshop moved; the new one is used daily. The past form closes the old situation, the present form names the current one.
Check your time reference first. If the sentence points to now, a habit, or a general truth, choose is used. If it points to a definite finished time, choose was used.