When you see a deal that mentions a loan assumption, it's likely that the existing loan has a pre-payment penalty that the seller does not want to have to pay.
If you get new financing then the seller would have to pay this penalty. Some commercial loans have this penalty which typically gets lower over time and after a number of years goes away completely.
To keep the seller happy, plan on working with the existing lender to assume the loan. This process is about the same as the process you go thru to get a new loan. This means you need to qualify, meet all the lender's requirements and usually sign personally on the loan.