I think of the phrase as signaling comradery and inclusion if the listener feels the same way. For example, often the socially happy response is to emphatically answer, 'me too! I couldn't live without my i-pod!'.

It is a bit of a low wage gamble, often submitted to the air, even among people you don't expect to agree. In these cases, the signal is that somewhere there is an in-group that feels this way.

I see it as a norm-thing, along the lines of, 'I'm biased about this and that's OK, because why wouldn't I be?'.

As Constant wrote, my experience is that it is meant for inconsequential personal preferences. If someone ever used the phrase for something that did matter to the listener, I think a simple appeal to the importance of the choice would be quite sobering.

For example if someone said, 'I know I'm biased, but I love my SUV', the statement could be gunned down with, 'I know I'm biased but I love my planet'. Of course it would be rude to do this in conversation, since the 'I know I'm biased ...' is foremost a friendly white-flag signal. (NB I personally have no problem with SUVs.)