Have troubles understanding the use of en

[1vladimir]

So I met this sentence : "Je m’en allai dans le jardin" which means i went to the garden.

I know that en can replace quantities or places or words like some and any, but Idont see a reson to put en in that sentence since it is already stated taht he is going to the garden. Wouldnt it be enought just to write "Je m’en allai"?

 

[Kirill]

Looks like this is a set phrase s'en aller:

 

https://www.multitran.com/m.exe?l1=4&l2=2&s=s'en+aller&langlist=2