We read code for all kinds of purposes other than to make an intended change. Like
How the heck does this behavior (good or bad, expected or not) come about?
Is this correct behavior assured in every case?
What are all the possible values of these poorly documented configuration parameters or other inputs?
Will this apparently working operation break under concurrency?
Why is this so slow for these inputs, or under these conditions? / Why is this so unexpectedly fast?
Is this doing nasty covert things I don't want, in addition to the overt functionality that I want?
Is this using a particular library or OS feature to do something obvious, or did they roll their own?
How will this scale in time, memory use or whatever for certain large inputs that are too impractical to actually supply just to answer this question?
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