Since when do you not need the actual error object in the catch clause? The example is one of a bad habit of making debugging hard by not telling what exactly happened. Maybe it timed out, maybe the connection was refused, or maybe it returned 404 but I guess the author doesn't care what the detail is making it harder to reach the source of the problem.
Not sure how this feature made its way in.
Depending on the specifics of how it's handled, you may or may not need the actual error message. For example, in production you may want to implement some sort of logic (retry, display friendly error message, etc.), none of which needs the actual error message.
This feature isn't unique to PHP. Other languages with Exceptions, for example C# and C++, have this exact same feature.
It's possible the language was just following usage rather than leading it. Regardless of whether or not it's right, I commonly see something like
} catch (\Exception $e) {
// no-op
}