Lubrication reduces friction, risk of injury and pain. This makes the use of condoms more pleasant because the materials used in condoms aren't as soft as our private parts, so using them involves more friction. A lubricant also relieves vaginal dryness. This can be caused by many things such as stress, alcohol, hormonal changes, the pill, etc... Vaginal dryness is characterised by insufficient genital hydration where the natural lubrication produced by the intimate mucosa is insufficient. This disorder can cause pain during sexual intercourse, weakens the mucosa and increases the risk of gynaecological infections. The use of a lubricating gel is not reserved solely for those who suffer from this but is a great help to everyone just generally π
There can also be what we call a mismatch in physical relations. This means that somebody can be excited without producing lubrication. This is normal and happens more than you might think. In such cases, it's much more pleasant to use a lubricant to support your pleasure.
Using a lubricant during masturbation will clearly intensify any sensations.
It also prevents the desensitisation of the clitoris and vaginal walls during friction!