Hey there @omer-f
Mainly it comes down to 2 things :
- How does the preset & internal effect sounds & sits in your Mix ?
- Workflow & routing and what you're aiming at with the reverb.
If you feel like a synth preset with an internal reverb sounds great in your Mix, then why not using it, but if you're after a more solid, mono"ish" sound or if you feel like the effect is too much pronounced or if you just don't like how the reverb sounds then you might want to disable it and use your own reverb.
When it comes to workflow & what you're trying to do with reverb, it might be more easy to automate an external reverb plugin than the internal synth one. That will likely depends on the plugin allowing internal effects automation or not, some don't or aren't that easy to map/automate.
If you're aiming at drastic reverb effects like for example a gradually washed sound for pads, then it's more easy to use an external reverb set to full on a return channel and automate your sends than an internal synth reverb.
But mainly it all comes down to how it sounds in the Mix in the end, do you feel that the internal reverb/effect is doing good to the sound or not.