Off-the-shelf software is often the right answer. It's quick to adopt and someone else maintains it. Custom software makes sense in specific situations.
When off-the-shelf fits
If your process is fairly standard and a well-supported product covers most of it, start there. Integrations can often close the remaining gaps.
When custom makes sense
When your process is a genuine point of difference, when you're stitching several tools together with manual work, or when customers need an experience no product offers.
The middle path
Frequently the best answer is a mix: existing products for the standard parts, with custom tools and integrations around what makes your business different.