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Grammar question: difference between へ and に particles

へ and に tend to be used the same.
But they are originally a bit different.

I went to the book store. 私は本屋へ行った
Explaining the fact that you went to the book store but this sentence is not talking about the purpose.

I went to the book store to buy a book 私は本屋へ本を買いにいった
Explaining the fact that you went to the book store to buy a book.

I went to the book store (to buy a book.) 私は本屋に（本を買いに）行った
Explaining the fact that you went to the book store, and implying you went there with a purpose.
Poeple can probably tell that you went there to buy books.

Another example,
I went to the bank (私は銀行[へ]行った）-> telling the fact
I went to the bank to withdraw some money. (私は銀行 [へ] お金をおろし[に]行った) -> telling the fact and purpose
I went to the bank (私は銀行[に]行った) = (私は銀行（へお金をおろし）[に]行った）-> implying there was a purose to get some money

But I would say those are pretty much interchangeable recently.