" I cant really fathom what i would use an inverter for or an adder for, i need to see it on an oscilloscope and hear it with my ears first."
The inversion won't make a huge difference to audio rate signals but will make a major difference in much slower CV.
Well, let's take an attenuverter module as an example. I own a Befaco dual attenuverter.
https://www.modulargrid.net/e/befaco-dual-atenuverter-
Each channel has two knobs. One controls attenuation the other controls offset. Full clockwise passes your original signal, full counter-clockwise passes your original signal 180 degrees out of phase. 12 o'clock mutes your signal. The positive range simple attenuates the signal, the negative range ALSO attenuates the signal but inverts the phase. The offset moves where your signal centers.
So let's take a simple LFO and plug it into the Befaco. Its range is -5 to +5v. If I start at full clockwise and turn the knob down I can diminish the range to something else... let's say -2 to +2V. If I'm in the counter clockwise range I can possible have my LFO running at +2 to -2V. Again, for slower modulation, this will make a difference.
Now let's get to the offset. We'll pick up at the +2 to -2V we just dialed in. By turning the offset knob I can keep the range of the signal... which is now 4V peak to peak versus 10V where we started... and offset it to something else. How about +5 to +1V, or -1 to -5V?
In the modular environment, it's all about the voltages. So being able to manipulate them in this way is going to give you much more fine control over how you modulate a CV input. Some modules do have built-in controls over the CV inputs; others do not. Some have controls for some functions but others are just a straight input (think of a filter with a built in attenuator for filter cut-off, but only a simple jack for resonance).
Precision adders. They simply sum a signal. But they do so with... precision. For something like summing two sources of modulation to control a filter cut-off... it's probably overkill. But where it counts is if you're taking two CVs that are controlling something like an oscillator's pitch and trying to sum them into a third pitch, you want something that's going to be exacting as inaccurate summing when you're trying to control pitch will lead to notes that are out of tune... even more so if you're covering a greater range of notes.
I'm sure there are details I'm leaving out and more accurate ways to describe the above... but in practical terms, this is why that these types of modules are very helpful in a modular environment.