Quote from LoroeseThat's kind of the right idea. Specifically, spells either use an attack animation, like FN and WW, or they don't, like DS and EB.
If they don't use an attack animation you can cast it at any time it is available, along with any spell that doesn't use an attack, so DS and EB can both be cast at the same time. If a spell uses an attack animation, that means you can't use another spell like that for 1/APS seconds. That also means you can only cast either FN or WW for any given attack. Again, you can cast DS and/or EB at the same time as either FN or WW because they do not use an attack animation.
One reason for higher IAS is to have more WWs active. With 1 APS you can only have 6 up a time (6s duration), neglecting AP cost. With 2 APS you can have up to 12 active. Also, the CM procs and LoH returns for WW increase with attack speed, which makes the build work better. Also, with say 3 APS you can use one attack for Nova and 2 for WW, so you can still have several WW active and still be able to cast FN frequently, plus the extra tics from WW refresh FN faster. DS can be cast at any time and constantly spammed without interuption of WW/FN casts.
Allright thanks, that explained alot and will make it easier for me to play!<3