Idol group rappers are like a subset of the rap genre. There is a fixed structure to idol groups that always includes a rapper in male groups and also in many female groups.
Because rap breaks are usually set apart from the rest of the song and focus on no more than two people the rappers tend to get more individual applause onstage. The distinct sections of spoken word versus singing are helpful for crowd participation and help make songs more memorable.
Some idol rappers really love hip-hop and work hard to write interesting lyrics and improve their rap skills. There are several very talented idol rappers with impressive technique and massive stage presence. Strong rappers are a point of pride for fans.
At times idols are cast in groups because of their dance skills or they might be very talented in other areas offstage, but if they don’t have a great singing voice then it’s sometimes easier for the agency to teach them a basic form of rap than it would be to teach them how to sing well. As a rapper they can still participate on the albums while focusing more on their other specialties.
Sometimes the rapper is also a good singer, but the idol group structure includes a rapper, so someone takes on the role. This type of rapper tends to end up singing more as the group’s career progresses after their role as rapper is established during the rookie period.
When an idol introduces her/himself as the rapper, that could mean anything from basic song participation to clever lyrics delivered with a powerful flow and wild vocal inflection. You need to get to know the group’s music to find out.