The pattern へ行(い)く is frequently used to indicate the direction of movement toward a specific location. While the particles に and へ can often be used in the same way, they carry slightly different connotations. に implies arriving at a destination, much like 'going to', whereas へ emphasizes the journey in that direction, similar to 'heading toward'.

• 田中(たなか)さんは図書館(としょかん)へ行(い)く。
  Tanaka-san is heading to the library.
• マリアさんは郵便局(ゆうびんきょく)に行(い)く。
  Maria is going to the post office.

Although these sentences appear similar, へ places a stronger focus on the path or journey taken, while に highlights the final destination itself. This distinction applies to other situations where に and へ are used. に consistently points to the endpoint of an action, whereas へ draws attention to the general direction of travel or the space between the start and end points.

• 友達(ともだち)へ手紙(てがみ)を送(おく)る。
  To send a letter to a friend. (Focuses more on the direction the letter is sent)
• 友達(ともだち)に手紙(てがみ)を送(おく)る。
  To send a letter to a friend. (Focuses more on the recipient of the letter)