If I owe taxes and going to payment option why do I have to put bank routing number

If you are going to request a payment plan through the IRS then just select the option to pay the taxes owed by mailing a check using the payment voucher.  Do not use the option to pay your taxes owed through direct debit from a bank account.  E-file the tax return and do not pay the taxes owed with a check using the voucher.

Or if you are completing the Form 9465 Installment Agreement Request as one of the options for paying your taxes, then you need enter the bank routing number and account number on the form so the IRS can debit your account for the taxes owed according to the plan requirements.