In the first instance most insurance companies and banks will require a survey especially on older vessels. They wish to understand the condition and ascertain whether the yacht/boat is a good risk or whether the bank is prepared to lend money on the yacht.
The more pragmatic reason is that knowing the condition of your yacht prior to offer and consequent purchase often saves you money.
If you buy without engaging a Professional Marine Surveyor:
However over and above the commercial reasons, the most important reason to survey your vessel is your crew safety.