It's common enough, and suggests that the following statement provides information that contradicts, emphasises or otherwise provides a strong insight on what has already been stated.
It's only really a filler if that meaning isn't required. (Some might consider it superfluous even when it is, and worth dropping, but that's another matter).
Which fillers people use tend to be very much influenced by region and sub-culture, so they are often "tells" about someone's background (those who use "like", those who use "innit", those who use "you know?" and so on). They are often adopted as shibboleths within groups to show people identify with the rest of the group. Teenagers are often criticise for using fillers that their elders dislike, but (1) teenagers have a particularly pressing need to signify that they identify with their peers and (2) those elders are probably underestimate how heavily they use fillers of their own, because such fillers become invisible to those who use them.
It's a good idea to avoid fillers generally, and even more so in writing than in speech, to focus on what is more important and to avoid alienating those who don't use the same fillers themselves. It is not something to worry too much about, though.