For vs to (Planing is important for/to long-term success)

[Ganesh]

When we have to use 'to'
and in what situation we have to use 'for'

1.Planing is important for long-term success.

2. Feedback is important to evaluation of outcomes to see if plans are being achieved

[Theresa]

It is adj + to infinitive--> It's a structure
for (prep.) + Noun
Both "to" and "for" can be used to express purposes but it is advisable to use Noun after for and Verb after to

[Cerberus™]

Whether to use "for" or "to" often depends on what word you use it with. In this case, it depends on "important". I'd say "for" is best in both sentences.

"Important" goes with "to [person]" to express who it is that finds it improtant.
It goes with "for [thing]" to express what goal it is important for, but "for" may also be used instead of "to" as described above.
As Theresa said, if you want to use it with an infinitive, you should use "to".