Leveling up an account in OSRS (Old School RS) takes many hours. In fact, most accounts have been trained for hundreds, even thousands of hours. This means that if you create your own OSRS account, it will take months to achieve the stats you want.
When you buy an OSRS Account, you save yourself all this time which you would have otherwise invested into training and leveling your in-game account. Some people enjoy grinding and achieving this by themselves, which is obviously perfectly valid. However, some people simply do not have time, or get bored by leveling and/or training. In that case, it makes perfect sense to buy a ready-made account instead.