As @Doug Smith had mentioned, it very much depends. If you are trying to purchase a home for all cash, then you won't need a preapproval. If this is the case, just explain it to the realtor/seller that you are intending on buying the property this way.
If you are planning on purchasing a home with a mortgage, then oftentimes realtors will not show you a home unless you are a verified buyer; A.K.A. preapproved.
Lots of buyers think they can afford what they are looking for, but often times they are misguided because of things they didn't realize were taken into account when applying for a mortgage. I'm not saying this is you, but this is why realtors don't like showing homes to buyers who are not preapproved.
If they are not guaranteed that you are verified to be able to afford the purchase you are trying to make and they put hours/days/months into helping you just to end up getting denied on the financing, that would be a great waste of time and money for both parties.
You don't have to get preapproved, but for realtors and sellers to take you seriously (unless you are a cash buyer willing to show funds) you should get preapproved.