Newby to this... might be silly question... why does one need multiple DAP’s? Is it to have different sound signatures? Different functionality? Different size?
Let's see here, my first player was a sansa fuze which I got from my mother back in 2012 as a birthday present. I rockboxed it about a year later because it couldn't play AAC files. (functionality upgrade)
2 years later, I got the FiiO X1 because the scroll wheel on the sansa fuze was dying. (necessary replacement). It was around this period where I really started getting into audio. While having the X1 I managed to try out the DX90 and got a taste of how a good player sounds like. Unfortunately I did not have enough money at the time so I couldn't get it. It did, however, give me upgraditis.
Another 2 years later, now in 2016, I used a SD card issue(some folders weren't showing up on the X1) as an excuse to get a new player. The Xduoo X3 (Sound upgrade). And just a year later, the X10 came out and I managed to pressure my father into letting me buy the X10 (Massdrop sale was ending, really good deal compared to what we could get here, gave the X3 to my younger brother)(sound upgrade). At around 6 months after my X1 had the SD card issue my sister's X1 died for real (wouldn't boot, just heat up like the sun when switched on) so I managed to convince my father to get her a Cayin N3 (I wanted to try out the N3 sound, it had the same dac as the X10, the X10 sounded superior btw)
Fast forward to 2018 I found out that my Xduoo X10 had degraded in sound after relistening to my sister's N3 and finding that the N3 sounded better. This lead to me buying the N5ii just last month (sound upgrade but as a result of not wanting a sound downgrade, UI and functionality got a huge upgrade too

)
Tl:dr on why i changed player : replacement player, sound upgrade, sound upgrade, sound upgrade-ish
So mostly it's because you want better sound.
In the case of audio players I think I would prefer to go step by step instead of immediately splurging on an A&K A&ultima SP1000 for example. You get to hear each upgrade. (As always, diminishing returns will apply)