Short answer
Because the software you choose should fit the way you want to work, not the workarounds you built around your old system. Agencies that redesign first write sharper requirements, run better demos and avoid paying to rebuild old habits inside a new product.
If you don't start from the bottom, and if you're only looking at replacing system A for system B, you're often stuck doing the same old workflows in a new product, and I have watched that happen to agencies that bought perfectly good software. Most of the steps in a government finance or HR process were added for a reason at some point, like a missing report, an audit finding from years ago, or a system that could not handle approvals, and many of those reasons no longer exist, so the redesign is really about asking which steps still earn their place.
The way I like to do this is to get the people who do the work in a room with a whiteboard, walk through how a purchase order, a new hire or a budget transfer actually moves today, and then ask what it would look like if you were starting from zero with modern tools. That conversation produces requirements written as outcomes and scenarios, which are far more useful in an RFP than a list of features, and it gives your team a shared picture of the future before any vendor tries to sell them one.
It also makes the implementation faster, because the configuration decisions that usually stall a project for weeks have already been made by your own people. The Start at Zero Navigator is built around exactly this sequence, and it is free to download.