Do we have a biologycal need to dream or it`s just a way for the brain to relax?

Many theories: Long term memory processes appear to be linked to dreaming. Therefore, if you want to have good memories then dreaming could be a biological need.

Yes, and no.: The brain has a mind of it's own. The brain 'relaxes' during the first 3 stages (non-rem stages). Each goes deeper into non-activity: lower pulse, respiration, restore mm conditioning and immune activity, active motor. Stage 4 (rapid eye movement) is the only stage we dreams, there is compete paralysis of mm, dreams reflect all unconscious perceptions. Cycle repeats about 7 times every night!