I am not sure that I understand the question here.

If you are asking "why would you use a divert valve". The answer is that they are most commonly used when a single conveying system is required to deliver to 2 or more destinations. To isolate the line to one silo when you are filling the other for example.

If you are asking why they are normally 45 or 90 degrees (there are some which are 30 / 60 degrees as well) by the way, the answer is that they are normally castings and are standardized at a convenient angle for piping design.

If you want a special angle for any reason, there are manufacturers who can make specials, or what many people do is just to fabricate a Y branch and put a full bore ball valve on each downstream leg of the branch. ■