Why should we always need to purchase original software?
The answer in simple words is IP theft and also malware. Basically when we install a cracked version, cracker/keygen modifies the software library which deal with the activation part and sends the fake signal to the trail software that it is activated. This cracker has to run in the background as a service inorder to reply the software because it will check for activation periodically. And the cracker has to run with high privileges.
Now the cracker software is running in the background with high privileges can also install other malicious software. This malware now can grab our keystrokes, search history, passwords etc.. and sends to the C&C server. It will be even worse if our system is part of the botnet. If a software has vulnerability and not patched then anyone can exploit that without even having technical knowledge.
Why should we install original?
Always original software come with the patches. They will release the patches the moment they receive the vulnerability report. Not only security patches but they also push lot of other bugs to make it work smoothly.
Finally, one pirated software is enough to compromise our entire digital world.