If Kouhai wasn't such a terrible person she'd be so peak.
The conditioners in 2 in 1s tend to be kinda trash.
Hair is dead protien. You cannot "heal" or "nourish" existing hair. The best you can do is try to coat the shaft in something protective and slick. You can "moisturize" hair to a slight degree, but mostly you are just adding oil to expand protein structures.
Conditioners work at the scalp. Some provide nutrients that are absorbed by the follicles, but most just moisturize and soften the scalp epidemis (so it doesn't run, sand-paper like, on the hair shaft. And your hair also doesn't rub on it) and provided afforementioned protective barrier to the hairshaft. This means you need to get them applied to the scalp, and leave them in place for several minutes so they can transfer/seep.
Basically the need to condition hair is determined by 3 factors:
1) Starting hair strength, that is how strong/thick your follicles generate the hair shaft, and the quality of the protiens. This usually degrades with age and is affected (primarily) by genetics and diet
2) Amount and quality of oil your skin generates. Your body generates oil to keep your skin from drying out. Too much, you are greasy. Too little, you are ashy. At pores with hair int hem, that oil is applied to the hairshaft as it grows out. If your body makes the right amount of the right viscosity of oil, you will need less conditioner. Brushing hair helps by helping transfer oils from the root of the hair shaft to the tip.
How much hair you have also affects this, since your body may not be great at upping oil production to cover more hair.
3) Environmental factors. Sun exposure, humidity, dirt/dust in the air, microparticles, sweat, even hair length and how much motion your hair recieves as the shafts will rub on each other. These can all reduce the effectiveness of your natural oils to keep the hairshaft coated and lubricated.
Soap breaks down lipids. A 2-in-1 will often have a "non-soporific" moisturizer, but it is usually of questionable usefulness.
tl;dr: 2-in-1s should be treated as a "moisturizing shampoo" not as a conditioner replacement. They are fine for short hair and people with good quality hair to start with.