> The problem this solves is is when you are using haproxy but need redundancy or a single server is no longer capable of handling the load and you need to scale out to 2+ servers.

Please describe how HAProxy running in l4 mode is a solution to this problem.

haproxy terminates tcp session. ipvs/katran does not. the whole purpose of ipvs/katran is to scale out layer of load balancers, which terminates tcp (transport in general) sessions. so tl;dr ipvs/katran solving the problem of scaling out "haproxy/nginx/varnish/w/e else" layer