Because it also enables threshold 2 to be triggered. Once threshold 1 is passed at a tick time, a trade will happen if threshold 2 is triggered on the next tick (or a later one, provided that threshold 1 is consistently exceeded). So basically you can set the bot to require a certain steepness(? no, that's not the right word) of the EMA diff change, or to just trade immediately when the lines cross (by setting T1 to a negative number and T2 to zero) or whatever you like.

So to clarify: if the EMA diff somehow jumps from a negative value to a large positive one so both thresholds are met at once, nothing will happen, because at first only T1 will kick in (unless it already has, see above).

Did that help?