Many businesses buy a managed rather than 'wires only' network, since this avoids the cost of the skilled resource required to manage the WAN and to reach every site.
For those who would otherwise prefer to manage themselves, it may still be preferable to buy a managed network from a provider who's prepared to work alongside you as you manage the rest of the estate. For example, a managed WAN provider might allow your LAN devices, servers and applications to be monitored while you manage them. In this regard, they might act as a triage point and pass non-WAN faults to your resolver group while maintaining contact and support. It can also be reassuring to have a managed service provider on hand for when intractable performance problems (or worse) arise.
It may be prudent to assess a provider's ability to manage some or all of your estate, and to inter work with your teams.