Like Pleasure said here, it is at least worth trying to understand how some base/patch works that someone else has implemented before porting it over to your own hack. This is partly to understand the process behind how they work, and thus learning something new.
Sometimes you may even want to apply some patch to your own hack that does something, but there is something missing with the patch and you need to implement that missing part yourself. Then knowing what the patch does may be crucial in being able to do this.
Another thing I can remember by personal experience is that sometimes the patch I want to apply is generally broken in some way, and I have to fix some bug with it. This is something you normally wouldn't have to consider though - especially if you apply a patch that is being used a lot and no one has run into anything broken with it.
But perhaps the most important reason why you should know what some patch does is to make sure it doesn't break anything in your hack when you port it over. I don't know about other patch formats except for IPS, but IPS for example uses the following format in general to define, what the patch does:
Code:
1. Address where to write to (3 bytes) to cover address space from 0x0 to 0xFFFFFF (3-byte max value).
2. How many bytes to write to (2 bytes).
3. Data... (as many data bytes as told in 2.)
and this part repeats over and over again (apart from a few exceptions).
Basically, IPS patches, but probably also other patching formats have predefined places where to overwrite data. So when you apply any patch to your own hack, it doesn't look for, "Where is some free space for this code/data?". Instead, if you have written your own, new stuff in an address where the patch is supposed to write some new data to, when you apply that patch to your hack, something will break down. Thus it is important to understand at least, where the patch writes new data to and at the same time, know what kind of data you have stored in that address yourself.
And if you want to apply some patch to your hack that would overwrite some new stuff you have written, you either have to move your own stuff out of the way so that the patch can safely write there, or hack the patch file to write that data elsewhere and change some pointers around in that patch file (which I personally wouldn't recommend doing in this kind of a situation, but this could be done).
What comes to your own questions though, I would personally use a patch (or base) made by someone else if it does something I personally can't do, or it would be a time-consuming process to implement by myself. But even so, I would make sure the patch clearly doesn't break anything in my hack when I port it over.
One handy tool for checking out what an IPS patch does is IPS Peek:
https://www.romhacking.net/utilities/1038/
I haven't checked if a similar tool exists for other patching formats, but that one is something you really want to use if you want to port some patch made by someone else over your rom file.