Beyond the most obvious reasons - that players are bored or that there are obstacles - there are actually a few reasons as to why many endgame PvP and PvE players often jump or "bunny hop" in The Elder Scrolls Online.
One thing to take into consideration while you read is that Jumping in ESO does not cost a Global Cooldown (GCD) so you can Light Attack, cast skills and so forth while you are jumping.
For PvE, particularly for tanking, you can actually aim your reticle over the mobs in front of you (depending on the size of their hitboxes) and target the mobs behind them or to the side of them.
This prevents certain mobs from changing directions or risking them hitting the group.
As a note there was a rumor that jumping up and down in place during Lokkestiiz Hard Mode's beam phase reduced the damage ticks of the static AoEs, this is not true.
The main reason for this is movement. Movement is very key to surviving in PvP especially when outnumbered.
Jumping can often disorient other players (by distracting them while casting animations making it harder) or make light attacks from other players miss as most players don't use tab target.
Jumping can help you get behind players particularly in duels forcing them to reorient themselves or change their camera focus or to kite away from them as you cast abilities.
Jumping also gives a clear indication of getting stunned, imagine you get stunned as you're mid-air, you will "feel" and know that you're stunned giving you a split second of reaction time.
On that note, you will also notice that if you get stunned or CC'ed while you jump you may also happen to reposition yourself a bit further away as opposed to sprinting and getting stunned mid-sprint which is advantageous against more people if they're chasing you down with AoE attacks.
Perhaps another reason for many players jumping in PvP is that it helps give them a tempo.