It’s not good enough to run cables and just hope they work, or simply say it’s all good if they provide a working network connection to the computer or device. You should double-check by testing or qualifying the cable runs before you call the job complete.
You should use a tester to check if all the cable pairs are intact and correctly wired and see if the cable can truly handle the data rates you desire. Network testers can also be a lifesaver when troubleshooting network issues or making changes to the wired network.
They could for instance tell you which cable pairs you might have mixed up when terminating the cable. Or if you’re working on someone else’s network install that didn’t document or label any cable runs, you can utilize the tester to help identify where the cables are running.