When a company that earns $150k/yr and hires a software engineer who costs $100k/yr, that company should be able to deduct that salary in full.
The way it works today they must amortize it, taking only 20% per year. So they’d owe taxes on $130k of $150k, instead of the more rational $50k of $150k.
This effort is to restore rationality.
Why was the change made to amortize the tax?
To bypass the filibuster.
To pass a bill the normal way, you need 60% of the Senate to agree. But certain kinds of bills can be passed through "reconciliation" which only needs a simple majority. Such bills must be "budget neutral" as assessed by the CBO. So legislators throw in hacks like Section 174 in order to game the system and offset other provisions they actually want.
Here's more information on the Reconciliation Process for anyone interested.
PDF: https://www.congress.gov/crs_external_products/R/PDF/R48444/...