Short answer
A strong implementation partner configures the system, converts your data, trains your team and brings people who have done this in governments like yours. An independent advisor sits on your side of the table to protect scope, quality and decisions, which matters most when your agency has never run a project this size.
The best implementation partners I work with show up with named, experienced consultants who know both the product and public sector rules, and they push back when your requests would make the system harder to upgrade, which is exactly what you want even when it is uncomfortable. They also run disciplined testing, give you honest status reports, and leave your staff able to run the system after they leave, rather than dependent on them for every change.
The challenge is that the implementation partner works for its own firm, and when a question comes up about whether something is in scope, whether a deliverable is good enough, or whether a delay was caused by the agency or the partner, you want someone whose only interest is yours. That is the role of an independent advisor, sometimes called buyer-side PMO or IV&V, and in my experience it pays for itself most clearly on large projects, on projects where the agency has a thin internal team, and on projects that have already started to drift.
If you do bring one in, make sure the advisor sells no software and takes nothing from the vendors, and set the roles clearly at the start so the implementation partner sees them as a help rather than a referee. That is how we work at Avero, and the Start at Zero Navigator explains how to set up that structure even if you never hire us.