Think of this a couple ways -- letting Siri work with less supervision, and how people usually read watches and timers.
I think, if I set a timer and kept looking at my watch (or stopwatch, or kitchen timer), I'm wanting to see if it's running, or reconsidering whether to change it to a different time. I wouldn't want the timer screen to disappear if I change my mind, then.
And, Apple has been giving Siri more hands-off capability. I've heard that we could tell it, "Hey Siri, text my wife that I'm coming home now," and even if I put my wrist down without doing anything more, it will send the text on its own. I tried it just now to send a text to myself, and doggone, it worked. For a timer, you can do pretty much the same thing.