What is the need of logstash when we can directly forward things to elastic search, is there any specific reason that we want to use logstash?
It's not unusual to want to process events in some way (renaming fields, performing DNS lookups, parsing user agent strings, ...). Logstash also has a wide variety of inputs and outputs that Beats (which I assume is what you're talking about) can't match. If you have no such needs then Logstash probably has limited value for you.
Like Magnus said, you can't do geoip and more processing via beats.