Sure, I'll give it a try:
If you buy a property and finance it with a fixed interest rate, the financing bank will have a land charge registered as security. This land charge is ranked first in the land register. This means that if there should ever be a forced sale, this bank will receive its money first (i.e., with priority).
A bank registered thereafter (in second rank) (which would finance the house) would then receive the remaining proceeds and may thereby be left with part of its claim unpaid.
Hence: this procedure makes the security for the second-ranking bank significantly less valuable, which will be reflected in worse terms.
If, on the other hand, the loan for the property purchase has a variable interest rate, it can be redeemed upon financing the house (then integrated into the house financing) and the first-ranking land charge becomes free again and can be used for the new financing.
Clearer now? ;)