Established orgs often have Lead Assignment Rules already set up. The Lead object can have one active Lead Assignment Rule active at one time. Whilst SuperRoundRobin works well alongside Lead Assignment Rules, there are a few things you should consider when implementing SuperRoundRobin in conjunction with Lead Assignment Rules. All of the below applies for Case Assignment Rules.

Lead Assignment Rules run after SuperRoundRobin...

When a record is created, Salesforce does things in a certain order. What is relevant here is that BEFORE triggers (such as the one employed by SuperRoundRobin) fire before Lead Assignment Rules. So if you have SuperRoundRobin rules and Lead Assignment Rules that are not mutually exclusive, you will find that the assignments you expected from SuperRoundRobin are overridden by the Lead Assignment Rules. An important concept to understand is that if the lead does not match any of the Lead Assignment Rule Entries, the Lead Assignment Rule will assign the lead to the Default Lead Owner (set in Setup > Feature Settings > Marketing > Lead Settings).

So the two things to consider: 

• don't have overlapping or duplicate criteria between SuperRoundRobin and the Lead Assignment Rules.
• always have a rule entry at the end where the Do Not Reassign Owner checkbox is ticked.

The other thing is that Web to Lead forms invoke the Lead Assignment Rules by default, so if there are no rules, it will assign the lead to the Default Lead Owner, overriding SuperRoundRobin. So if you use Web to Lead with SuperRoundRobin, you must always have a Lead Assignment Rule with an item that has the Do Not Reassign Owner checkbox ticked (that's rule entry 3 in the image above).

Furthermore, if your leads are set to always 'Use Active Lead Assignment Rule' then you need to have that rule entry with Do Not Reassign Owner checked.

If you need to stop the Lead Assignment Rules from re-assigning leads that you have assigned in SuperRoundRobin, you need to have a rule entry like this (with Do Not Reassign Owner checked):

Lead Assignment Rules are useful if you want to use Queues

SuperRoundRobin is a bit opinionated and doesn't let you assign to queues. This is because we think queues are a step back - leads and cases go stale and forgotten in queues. But some orgs have to use queues for one reason or another. So we recommend you use Lead Assignment Rules to cater for any queue assignment needs you have. In the image above, the first rule entry matches when the Company is Acme. In this case, the company has a bunch of Acme Specialists in a queue of that name. I don't know why this company would want to do this, rather than have a SRR MatchGroup for Acme Specialists where they get the leads assigned directly to the queue members in real time, with notifications, and with the ability to vary the distribution or cap their leads. But hey ho.

Also you might have a queue that you've created to catch any leads that don't match on anything. See rule entry 2 - that matches when the Use Round Robin field is NOMATCH. That will assign to a lead queue that you've set up for this purpose. What I would be doing here is understanding what these leads are and making sure I create a MatchGroup asap that immediately assigns these leads to exactly the right people.