使用的是ajax.ashx标准吗？

I have inherited a existing website with a large codebase. To handle ajax requests, the site has just one file called ajax.ashx. Inside the file is a switch statement that looks like this:

switch (_json["m"])
{
    case "editDetails":
        if (requestIsValid(context))
            _resp = AjaxMap.editDetailsPro(_json);
        break;

    case "addNewContact":
        if (requestIsValid(context))
            _resp = AjaxMap.addNewContact(_json);
        break;

   // ... and so on.
}

There is about 50 different cases.

For me, this is not the natural solution - I would probably have a different handler for each group similar requests.

I haven't worked with web technology much before. Is this the usual way to do it? And if so, what are the benefits?

It is perfectly OK.

You save page lifecycle overhead that usually occurs when using Ajax in its default WebForms facilities (UpdatePanel etc.) or even over proxies such as WCF / WebService.

Not to say that these infrastructures are useless, they are certainly not. But for the sole purpose of getting and request, do something and sending net data response - it's fine.