Banks charge transfer fees from SlicePay for UPI transactions. To meet this cost and other overheads SlicePay charges a small amount as transfer fees to serve you better.
This transfer fees is still lower than the fees being paid by you to Paytm for transferring money from Paytm wallet to your bank account. Hence transfer to bank account through SlicePay is more affordable than Paytm recharge.
To know the current transfer fees being charged please check this - https://help.slicepay.in/hc/en-us/articles/360009591493-How-is-the-order-amount-calculated-for-bank-transfer-