Most buyers do not have enough cash available to buy a home, so they need to obtain a mortgage to finance the purchase. Since you will probably make your purchase contingent upon obtaining a mortgage, the seller has the right to be informed of your financing plans in order to evaluate them. That is one of the major reasons that financing details are included in your offer.
As part of your offer, you will need to disclose the size of your down payment. Once again, this allows the seller to evaluate your likelihood of obtaining a home loan. It is easier to get approved for a mortgage when you make a larger down payment. The underwriting guidelines are less strict.
Another reason for including financing information in your offer is to protect yourself. If interest rates suddenly become volatile and rise quickly, as sometimes happens, you may looking at a mortgage payment much higher than you anticipated. By putting a maximum acceptable interest rate in the offer, you are protecting yourself from such an occurrence.
At the same time, the seller will probably want to see that you have some flexibility in the financing terms you are willing to accept. If interest rates are currently at eight percent and you indicate this is the highest rate you will accept, you would be able to cancel the contract without penalty if interest rates rose past that point. The seller would suffer because they have lost valuable marketing time and may have made their own plans based on successfully closing the transaction.