Sequence order is somewhat similar to WBS number that some other application have. It is used by the tree view (mainly the work plan) to know what order to display the lines.
This field was always part of Clarizen, you just couldn't really see it prior to Clarizen v6. You can see it more in Clarizen v6 as when you open the Work Plan field selector, you can see the fields from the hierarchy link which describes the relationship between a task and it's parent and its sibling tasks.
Functionally, there's really no real benefit an end user can get from it and it can't be edited.