What
@Colin_T said and also it is supposed to focus more energy on the new roots. This is actually my first try at this and I am excited to see it work.
The plant cutting doesn't put all it's energy into roots. The first thing it does is waste a heap of energy repairing the damaged tissue where you cut the leaf. Then it starts trying to grow roots.
If you want cuttings to be more successful, there are a few things you can do.
1) Trim any lower leaves & branches from the stem/s you plan on cutting. Do this a few days to a week before you take the cutting. This allows the plant to heal the injured tissue while the stem is still on the plant. The cutting won't have to waste energy on repairs and can go straight to root growth and development.
2) Water the plant really well for at least 2 weeks before you take any cuttings. This ensures the cuttings are well hydrated when you take them.
3) Fertilise the plant regularly for at least 1 month (preferably 2) before taking cuttings. The plant will have stronger healthier stems and leaves and more nutrients will be stored in the cutting when you take it, thus allowing for faster and better root development.
4) Try to take cuttings when its cool so they don't dry out before being potted up. This is more of an issue if you take lots of cuttings and it can be a while before they get planted in some growing medium. If it's going to be a long time (many hours) before the cuttings get planted, wrap them lightly in wet paper and put them in an esky (cooler) so they stay cool and damp.
5) Take cuttings during the middle of the day so the stems and leaves have lots of nutrients in them. Most plants draw water and nutrients out of the leaves and stems at night and store it in the roots. When the sun comes out, they pump the water and nutrients back up the stems and into the leaves. This takes an hour or two, so letting them wake up and spend a few hours photosynthesising before taking cuttings, gives the cuttings more nutrients to live off.
6) Have the pots of cuttings sitting in a shallow tray with some liquid plant fertiliser. You want about 1/4 of the pot in the liquid. Use the fertiliser at 1/4 to 1/2 strength. The liquid will help keep the potting mix moist and the nutrients will wick up the potting mix and help the cuttings develop roots faster. You can also pour this liquid fertiliser over the leaves and soil and the plants can take in nutrients through their leaves. Some people use a spray bottle to mist the leaves with liquid fertiliser.