Most legislation that congress passes today is a mixed bag of special interest service, ‘pork’, and then a few necessary policy changes.
A line item vote would entail breaking down legislation into its severable parts, and allow voting on changes independently.
When you ask congressional representatives why they voted for or against a bill, typically they’ll say something along the lines of ‘I loved most of it, but this one part is unconscionable.’ This change would allow them to go on the record supporting, and hopefully passing, the parts of the bill which they support without having vote in favor of other (often unrelated) parts.
This policy would also work as a supplement to other legislative reforms such as single subject legislation; even with single subject bills there can be multiple changes with varied desirability.
Line Item Voting ensure that all new legislation has passed on its own merits, not just that it was packaged with other more deserving changes.