If you want to create arrays in legacy format, you would use --build. This may be because you have to connect this array to an old system which lacks metadata/superblock support.
You most likely want to use --create for common use cases.
To clarify the confusing text:
As there is no metadata/superblock on the disks, arrays created by --build cannot store any additional data about the array.
If you choose to work with arrays without metadata (by using --build) the applied operation is the same even if you have brand new disks or you have a previously used array. The lack of metadata prevents any possible detection.
In arrays with metadata, you create a new array on new disks with --create and subsequent uses of the array is conducted by --assemble, which uses the metadata on the disk to verify status of the array and correctness of the assembly.
Lacking the distinction with first time initialization and regular assembly process, --build option would create an array when you first call it on some new disks. Then you probably use the disk and write some useful data on it. On your next --build action, you expect to see the previously recorded data after the build operation, so you hopefully find some useful data on the array in succeeding invocations of --build
