Why are some points shown on a STAR but not attached to the route?
Coming from Southern California, you'll often be filed to one of those points, and then on to SERFR for that arrival.  Putting those two points on the page gives perspective of what direction one is coming in from.

It's all RNAV stuff, so is having them on the chart strictly necessary in order to navigate? No.  But it gives a little bit of a picture of what you're expecting to see on the Navigation Display.

Also, as mentioned in the comment, if ATC needs to reroute somebody off of what they filed &amp; over one of those points &amp; then to SERFR, having the point on the page may reduce copying errors when the controller spells out "cleared direct MAKRS, Mike Alpha Kilo Romeo Sierra" -- it at least gives the pilot something to reference when that clearance is being read to him.  Things that reduce the frequency congestion ("Was that Mike Kilo Alpha Romeo Sierra? I don't have that in the database..." "No, it's Mike Alpha Kilo Romeo Sierra" "Oh, okay, Mike Alpha Kilo Romeo Sierra, we'll use that.") are beneficial.