If they never hit you then you don’t need it
Constitution is vitally important. More durability for all classes (essentially a bigger health bar), and as a spellcaster, a better ability to maintain concentration on spells. There are other benefits but those are the biggest ones.
However, dumping con is an incredible role play opportunity. You could equate having low con with a frail or infirm person - someone who gets sick easily, can’t eat exotic food and spices, or gets drunk off of one drink. It could be a child adventurer. It could be someone with a disease who is on a quest to find a cure, or a magical item that increases Constitution (I’m drawing a blank on the name of it atm).
Your player dumping con is giving you the DM an incredible sub plot to go along with your campaign.
Edit: wrote “in” and corrected it to “I’m”.
magical item that increases Constitution (I’m drawing a blank on the name of it atm)
Manual of Bodily Health? (Do stat tomes exist in modern D&D? I don’t think I’ve heard of them since 2e.)
There are tomes still, but they are a permanent stat increase of +2 and are one time use only. Or, more accurately, useable once every 100 years, if I remember correctly.
I looked up the item I was thinking of and it’s the Amulet of Health (sets Constitution to 19). If the con is dumped, it’s likely 8, so equipping the item brings con from 8 to 19 and this gives the character +5 HP for every level they’ve achieved.
In other words, a level 5 character with a con of 8 would see their HP increase by 25 when equipping the amulet. A level 10 characters max health would increase by 50. It’s pretty significant, especially on squishy mage types.
The more Constitution the party has the more leeway your DM has with how deadly they can make their encounters.
This can also be solved by in game buffs but coming to the table with a little heft is still hepful.
