details of BigIP irules

I am not sure how this would help. I am not an F5 employee but this is my understanding. F5 came into existence in 1996.

2004-05 was when 9.x code version was released and I believe the iRule was provided as a feature from this code version. Before that the F5 code version was 4.x - there was a jump in number of major code versions (from 4.x to 9.x) because of the increase in feature/function.

iRule provides granular & flexible control of traffic flow. I would assume that before iRules, simple settings like the load balancing algorithm and persistence would determine the packet/connection distribution. You can also use "LTM Policy" in place of iRules in the 11.4 code version and later. "LTM Policy" is the replacement for formerly "HTTP Class". I prefer iRules to Policy but if you are new to scripting or don't want to script, Policy is a good alternative.

F5 in general and iRule/Policy specifically provides a single point of change in your environment. Instead of changing redirect information in dozens or 100s of servers, you can make the change on the F5 that is client-facing. I would say it reduces OpEx & operational/change burden.

If you are trying to newly install an F5, you should be on 12.1.1 code verison or at least 11.5.4 as the 10.x code train will be end-of-support by the end of this year.