Ovens are often inaccurate by as much as 60 degrees, so a probe suspended in the oven really helps dial-in & monitor the temperature; I've also noticed that as it cycles on & off, the temp can swing as much as 50°F above & below the setpoint.
This doesn't matter so much with something like a pork shoulder, but when baking a cheesecake or something where the temp needs to be consistent, the probe in the oven helps immensely.
Another probe inserted in whatever meat, cake etc, really teaches you what a done item looks like & how often its unnecessarily overcooked.
It's a reasonably inexpensive piece of equipment, so for $20 you can elevate your food from just ok to excellent.
You put all that work into preparing a food, you deserve the best results for you efforts.