Between 1989 and 1997 the 'Williams-Renault' binomal gave birth to some of the most dominant cars Formula 1 has ever known. Combining the power of the Renault engine and the genius of Adrian Newey those cars were meant to go down in history and so they did.
Four driver titles and five constructor championships were put in the bag in nine years only. Today, of course on a different level of performances, Williams and Renault seem likely to get back together, not to win tho, but to try to improve eachother's situation.

RENAULT'S PROSPECTIVE

The french team has two main problems at the moment: lack of performance and no avaiable seats for its junior drivers.
In the last two years Renault as a supplier has lost both Toro Rosso and Red Bull, which played a key role on the engine development. Losing those two teams imply losing a huge amount of kilometres useful to collect datas and therefore slowing down the development process.
In addition, the Renault Junior Academy has a plethora of young drivers that are difficult to place in Formula 1: there's no junior team for the french constructor and the only other car they supply, McLaren, is an indipendent team not willing to give a seat away.
Jack Aitken, Guanyu Zhou and Anthoine Hubert in Formula 2 and Christian Lundgaard, Max Fewtrell and Ye Yifei in Formula 3 may therefore not find a way to the Circus.
An agreement with Williams tho, may help Renault to solve both the problems.

https://www.motorlat.com/notas/especiales/11959/could-george-russell-be-mercedes-next-big-star

WILLIAMS' PROSPECTIVE

Just nothing to lose. Nowdays Williams is a team in a deep financial and tecnical crysis. They have to stand on driver's sponsors' money and it explains line-up choices of the last couple of years: Stroll, Sirotkin and Kubica are drivers with big companies behind and George Russell's presence let them have a Mercedes engine almost for free. Still they are possibly the only backmarker team of the 2019 season. For sure Mercedes' power unit has way better performances than Renault's one, but what do they have to lose? It would be an understandable gamble with a small chance of paying off but that for sure won't make the situation any worse. Even Paddy Lowe, directly sent from Struggart to improve Williams' performance, didn't get any result. Maybe new engeneers from Renault will help instead.

MERCEDES' PROSPECTIVE

In the last years Mercedes could count on both Force India and Williams to place its young drivers. Now the rebranded Racing Point cars belong to Lawrence Stroll, who doesn't need any discounts on the engine supply and whose main goal is to give his son Lance a seat. With a Williams-Renault agreement two more seats would be lost and Sergio Perez' one would be the only negotable left. Still Checo is a fast and experienced driver, with big sponsors behind and a quite good relatioship with the Stroll family. Mercedes' worries become therefore understandable: where could Russell, Ocon and possibly Wehrlein go then?