In the military (20 years), my motto was "over-prepared means fully-prepared". Why leave it up for question?
The US government wants to ensure the petitioner/sponsor can provide adequate financial support for the new immigrant after arriving in the US. Support comes income generated over the following year (current annual income) after entry, regardless of what the petitioner/sponsor earned last year.
What if I made $300,000 last year, but since filing my taxes, my income has dropped to $15000? My tax forms do not reflect my current income.
That's my rationale for including current income evidence.