It matters if you care about being accurate, and having a reasonable definition. It's the same reason you don't include suicides in figures about gun violence if you're actually interested combating gun violence and not just scoring political points.

Half the definitions at https://en.wikipedia.org/wiki/List_of_mass_shootings_in_the_... don't even exclude gang violence, drug violence, or organized crime. Statistically if you're not involved with gangs or drugs, you're not going to be involved in a gang or drug related shooting. The majority of those killed or injured are at least peripherally involved. So you should immediately question to credibility and motived of any organization that wants to talk about "mass shootings" which by definition are scary because they're random and unprovoked but then want to include a huge number of incidents are driven by crime, drugs, and gangs, none of which is random or unprovoked.