そのために

そのために is a connective phrase that links two sentences or clauses, referring back to something previously stated. It has two main uses: expressing cause/reason ("for that reason, because of that, hence") and expressing purpose ("for that purpose, in order to do that"). The meaning depends on context.

Cause / Reason – "for that reason"

When used to express cause, そのために explains that a result or consequence occurred because of a previously mentioned situation. It is similar to だから but sounds more formal and written.

In this usage, そのために points backward to the cause and introduces the resulting consequence.

Purpose – "for that purpose"

When used to express purpose, そのために indicates that the following action is done in order to achieve the goal mentioned before.

In this usage, そのために points backward to a goal and introduces the action being taken to achieve it.

Broader ために pattern

そのために is a specific instance of the broader ために pattern. While そのために uses その to refer to a previously stated sentence, ために can also attach directly to verbs or nouns:

• Verb (plain form) + ために:
  しけん試験にうかる受かるためにべんきょう勉強する。

  I study in order to pass the exam.
• Noun + の + ために:
  かぞく家族のためにはたらいている働いている。

  I am working for the sake of my family.

Distinguishing cause vs. purpose

A key way to tell the two uses apart: if what comes after そのために is something the speaker intentionally does, it is likely purpose. If it describes a situation or outcome that naturally followed, it is likely cause.