Thematic relevance of credits when icebreaking
Imagine a team of criminals planning to rob a bank. They need money to: 


Buy and maintain equipment
Conduct dummy runs
Bribe corrupt police officers to get information or turn a blind eye
Upgrade the getaway vehicle
Pay for fuel/food/cool hideouts etc.


In my mind, similar espionage costs apply to the runner. Let's take the icebreaker Yog.0 as an example, which has the fluff text:


  The Yog.0 database is a crowdsourced compilation of sniffed, spoofed, and logged passkeys. If the key to the gate is in the database, you're in. If it's not, change the gate!


Pumping up the strength represents that the gate you've come too is causing trouble and you need to 'emergency crowdsource' a key (read as 'pay some shady guy to give you the key for this gate'), or similar workarounds.