A personal payment is a payment to friends or family, someone you know. There is no buyer or seller protection on these payments because they are not supposed to be purchases.
A purchase is literally that, you are buying something, either from a shop or an individual. You probably don't know them and, as a result of this, there is going to be a level of risk inherent in making or receiving the payment. Both buyer and seller protection will be available (along with their normal limitations).
The fees structure is different as well, see below:
We charge fees when:
You receive payment for a sale.
You send or receive payments to/from another country or region.
Personal transfers are sent using a credit card (the sender decides who pays the fee).
Transaction fees can range from 1.4% to 3.4% + 20p. If you receive a payment from outside the UK, an additional cross-border fee of 0.5% may apply.
It's free to pay for a purchase, or make a personal transfer using money from your PayPal balance or your bank account.