Security clearances is something that qualifies you as an individual. It means that you have been evaluated and trusted to a determined security level.
If you are working at Company A and get a security clearance L1 and you stop working there, you do not lose the security clearance.
Of course, this does not mean that you can just appear at Company A and casually ask to see their sensitive files; since you no longer need to access that data to do your job you are not allowed access to the information.
If you then see a job offer at Company B that requires a L1 security clearance, then you can apply and "use" the same security clearance and use it to access to the information related to the new job.
You can repeat until the clearance is revoked.
This approach has several advantages:
Once cleared, if you want to change jobs you know beforehand that you qualify for any job that requires your security clearance.
The government avoids checking you again and again. Since the procedure seems rather complicated and slow, that helps a lot.
In case a former employer needs to contact you for something related to your previous work, you are already cleared.
At the workplace site there are some questions about security clearances; this one gives more details.