It's done for various reasons; some legit, some not.
Here's a few examples of why you might do this.
1) Your marketing department publishes different numbers for different campaigns and needs to track calls to each campaign, but the logic is the same regardless of number called
2) You want to have a number which bypasses certain menus or inputs and gets straight to connecting to an Agent (like a quick transfer into a queue), and so you create these numbers for that purpose and in the scripting check for which number was dialed and then jump to the corresponding section of the script
3) You have different languages you support but the logic is the same, and so you have like an english number and a spanish number and you simply switch the language of the trigger, causing a different set of prompts to play inside the script
4) Your doing testing so you put up a test number on the app which only you know about, and inside the script you have if condition checks for that being the called number, and when it is, it does some extra bit of code which you would like to test.
And there's more reason beyond that, but those are just a few off the top.
Also, if the numbers are contiguous, like 2000, 2001, 2002, etc., you can actually use a single wild card trigger like 200[0-2], instead creating them all individually. This will de-clutter your configuration and make your CM Telephony > Triggers page load much faster. However, you cannot use Called Number in your scripting anymore, and instead will need to switch to Dialed Number; one holds the trigger and one holds the number dialed respectively.
