Sivir is a range physical damage dealer and good in supporting the team. Her ults is very important because it will give her and her team movement speed. This is good in chasing enemies or engaging team fights. She also excel on pushing minion wave and clearing it effortless.

Pros:

Ult AoE movement speed

Easy wave clearing

Blocks first abilty cast by enemy to Sivir

Cons:

Very squishy

No dash or escape abilty

My Sivir main combo is auto attack, W(Ricochet) auto attack reset then Boomerang Blade. After that just auto attack alot, if the skill is available again just do it again. Important that you know when you will use spell shield. Spell shield is a very important ability to block first abilty cast on you. There is some multiple combo created by some champion that if they cast the same time you can able to block it all. Her ult is very important in team fight because it gives her team bonus movement speed. Useful at team engaging or chasing enemies