I know it raises TNOs healing rate, but why raise it? I have read that he regenerates back to life upon dying, and also that by dying he regains memories. So why do I want to prevent further deaths by increasing my health if the deaths can be beneficial? Please asplain. Thanks

While dying in specific situations can provide benefits, the rest of the time you still want to be able to win the fights you get into, and trying to do this can get rather frustrating if TNO is getting killed over and over.

Once you get to 18+, you'll start regenerating a hitpoint every few seconds, which can really add up, especially in longer fights or in areas with lots of enemies where you can't rest. There's also only one healer in the game, and if you don't have her in your party, you're going to need it. It also helps to take hits on TNO since he can easily heal them, and your squishier teammates can beat up bad guys in relative safety. Faster regeneration helps with that quite a bit, and you won't have to chew as many blood charms.

As for the dying, there are a few times where you'll need to die in order to advance the story. However, you do NOT want to go around dying in combat. Not only is it annoying (since you restart in the mortuary) but dying too often will actually make the end dungeon harder, for reasons that you'll find out later. Death is cheap, but you still shouldn't take it for granted.

Higher constitution will also give you more hit points. This means you can keep fighting a little longer.

An aspect of con that very few people discovered (It's not in Dan Simpson's very complete walk through for example) is that starting with a high con will give you more hit points at the start of the game. You start as lvl3 fighter which gives you 3d10 hit points at the start but there is no randomness to your actual starting amount of hit points. Con 9 will give you 20 hit points at the start, Con 10 will give you 22, Con 11 gives 24 and so on until Con 14 which gives you 30 starting hit points. Raising your Con further gives 3 points for each increase but that's just the regular AD&D rules.

It's also a matter of playing style. Do you want to be the vulnerable wimp who is even afraid of standard respawning street thugs and has to hide behind party members who fight for him or do you want to lead, walk in the front line protecting your party members from harm ?

I tend to work as hard as I can to get every possible interesting bit of dialogue in this game, so Constitution tends to remain a 9 for me. Strength gets raised by becoming a Fighter just in time for level-related bonuses, Dexterity gets fourth-string priority so that I can benefit from certain scenes requiring a bit of it, but Wisdom, Intelligence and Charisma get top priority, in that order.

Needless to say, I tend to keep him in the back. Which means magic, as much as possible.
It's all about your approach.

Actually, some dialogue options depend on other stats than Int, Wis and Char. I don't remember any particular option that was accessible only with high constitution, but I think several dialogue choices appeared only if you were strong enough. The game still has some surprises left in store for a player. :-)

From a guide I found in GameFAQs:

- Constitution: a very important attribute, that is worthy of being boosted
using the points that N.O. gets each new class level.

Every score between 15 and 19 gives you a bonus, which retroactively
increases your hit points, then again from 21 to 24, but from the score of
20 every character begins to regenerate his/her own life points (higher the
stat, faster the regeneration rate), which means that Nameless One do it even
faster.

Note that there is a ring that increases it by +3, so a score equal to 22 would
be a very good goal when planning your character customization, but it even
depends by your class: if you are a mage, you may want to keep hands free
for magic rings that boosts your spellcasting abiltiy.

Note that, with such a constitution, Nameless One is really immortal, since
no foe is strong and fast enough to overcome his regeneration rate.
:D

My character constitution is almost maximized (23), and he rarelly dies. *SPOILER*




Each time The Nameless One dies, another person in the multiverse dies to fuel his resurrection. These dead turn into ghosts that seek revenge on him.