Skill points, stress and morale requirements
Every so often your dupes will get skill points. The downside to adding a skill point to a dupe is that it will increase their morale requirement.
When you are in the skill tree menu you can mouseover skills to see the morale requirements unlocking that skill will result in. Look at the green and red bar under the picture of the dupe in the top left. Green is the dupe's current morale, red is the morale requirement.
If a dupe doesn't have enough morale to meet their morale requirement they will get stressed. If a dupe's stress reaches 100% they will have a stress reaction. What that is varies depending on the dupe, but can be things like vomiting or destroying nearby buildings. (You can see a dupe's stress response if you select them and look under Bio.)
After having their stress reaction their stress will go down to 60%. However, if their morale requirements aren't met their stress will start going up again.
Morale requirements and skill points is where the true significance of interests comes into play: having an interest in an area lessens the morale requirement increase for adding skill points in that area.
Sadly, the morale requirement increase from skill points means you can't have a small group of super-dupes that are all capable of doing anything you throw at them. Their morale requirements would be so high that they would constantly be stressed out and would, rather than being capable of doing absolutely everything, instead be largely incapable of doing anything at all.
So add skill points sparingly, particularly while you're still new to the game. Whatever a dupe is meant to be focused on doing, add skill points in that area. After a while your dupes will have more skill points than you can (or will want to) use. That's normal.
In addition to adding skill points in the tasks a dupe will be doing, I usually add two skill points in supplying to all my dupes. Supplying increases their carry capacity and is generally useful for any dupe carrying anything. (Which usually is pretty much all my dupes.)
Planning ahead and small dupe counts
Once you are somewhat comfortable with the basics of the game, and what your dupes will be up to as far as tasks and such, you can start choosing dupe interests based on more advanced, or "later game" requirements. (Meaning, if you have only just started playing the game, don't worry about the stuff I will cover below.)
If you are going to have a big enough dupe count that you can have dupes specialized in all the different areas you need dupes in, then these things don't matter much. If you want a smallish dupe count with dupes specialized in several things then it's something to think about.
For instance, if you want a dupe with maxed out digging & construction, choose one with interests in both those things. Maxing out construction is required if you want to deconstruct so-called Gravitas buildings, which are pre-existing structures in the game. Otherwise spending skill points in construction is only marginally useful, as dupes will get better at it from building things anyway.
Another example is having a so-called mechatronics engineer, which is needed to build some of the more advanced automation stuff. Unlocking the mechatronics engineer skill requires maxing out both supplying and operating. So having a dupe interested in both those skills will significantly lower the morale requirements of a mechatronics engineer.
You might want a digger & builder that can also build automation stuff, so they will need to unlock mechatronics engineer as well. In that case interests in digging, supplying and operating will cover all necessary skills.
(Something to keep in mind about skill points in digging: the fourth skill point, Hazmat digging, is only necessary to have if you plan on building nuclear reactors and then have them explode. If not, that fourth skill point in digging just adds unnecessary morale requirements so I'd say don't take it.)
A special case scenario, relevant mainly if you want a really small dupe count, is that you might want to have a dupe that can handle all the digging and building tasks you throw at them, including deconstructing gravitas buildings. For such a dupe, consider interests in digging, building and operating. You will need to max out supplying to get mechatronics engineer, but supplying only needs two points to max it out.
Such a dupe will have very high morale requirements, so it's a good idea to have good decor in their bedroom (carpets and paintings will do the trick), and somewhat decent food. But if possible, skip the interest in construction and put it in supplying for that dupe instead, as construction is only necessary if you want to deconstruct gravitas buildings.
