に行（い）く is a grammar construction that is used when someone is going somewhere for the purpose of doing (A). (A) representing the verb that comes before に行（い）く. As with other uses of に, this particle is highlighting an end point or goal of ‘going’.

• 釣（つ）りに行（い）く。
  To go fishing. (Fishing is the purpose/goal of going)
• 今（いま）からトレーニングをしに行（い）きます。
  I am going to go workout now. (Working out is the purpose/goal of going)

This grammar point is regularly translated as ‘to go in order to (A)’.
If you have already ‘gone’ somewhere for the purpose of doing something, then you would use ‘(A) に来（き）た’, while at the location.

• さっきあなたの友達（ともだち）が宿題（しゅくだい）を渡（わた）しに来（き）たよ。
  Your friend came to hand in their homework earlier.