What were the flares being used for in Écoust?

Question

In 1917, during the scene where Schofield travels through Écoust at night, there are white phosphorous flares being shot over the city, which helps to light his way.

It appeared that this village is still being held by Germans (albeit not many of them) who are now effectively behind enemy lines, as the Central powers had retreated, and the Allied powers were advancing. However, they were not under attack at the time, and clearly weren't expecting to be, as many of the soldiers were asleep or drinking.

Who was shooting the flares over the city? And what was the purpose of shooting so many of them? Was it some sort of signal, or was it to light the city in the darkness?

Answer 1

As you have mentioned,

It appeared that this village is still being held by Germans (albeit not many of them) who are now effectively behind enemy lines, as the Central powers had retreated, and the Allied powers were advancing

British soldiers were not completely sure that Germans were retreating. That's why they even held lines from where Blake and Will took off. If they knew they would had crossed those lines and whole regiment would had arrived at that village.

Flares were shot to show the presence of forces. As you might have noticed that even Blake shots a flare back to the direction where they came from to signal that they were safe (as instructed)

In my opinion and also because it seems tactical, Germans were shooting flares to show a strong presence in that area. Also the lighting up helps to identify intruders if there were any. As the soldiers of both the sides knew that enemy is shooting flares to get good view of them, so they would not dare to get out in dark.

German's presence can also be justified by reasoning that they were left behind soldiers or might be they had orders to hold that village. It also provided them cover from fire as well as weather.