It's not a legal requirement, and most tenants in Jomtien rent without it, but it's worth considering depending on what you're bringing into the unit. The building itself and the landlord's fixtures are normally covered by the landlord's own insurance or the condo's building policy -- that's not something you need to duplicate.
Where renters' insurance can be worth it is for your own belongings -- laptops, cameras, and other valuables aren't covered by the landlord's policy, and a straightforward contents policy is relatively inexpensive for the peace of mind. It can also cover personal liability, which is useful if you're ever responsible for accidental damage beyond normal wear and tear.
If you're only staying a few months and travelling relatively light, most tenants decide it isn't worth arranging. If you're settling in long-term with valuable belongings, it's a reasonable thing to look into -- a few insurers in Thailand offer policies aimed specifically at expat renters, and it's worth a quick comparison rather than assuming you need the first option you find.