dafür
in favor
Expresses agreement with or support for something; the opposite of 'dagegen' (against it).
Sollen wir ins Kino gehen? Ich bin dafür.
Should we go to the movies? I'm in favor.

for it / in favor
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in favor
Expresses agreement with or support for something; the opposite of 'dagegen' (against it).
Sollen wir ins Kino gehen? Ich bin dafür.
Should we go to the movies? I'm in favor.

'Dafür' (for it / in favor) is a pronominal adverb made up of 'da' and the preposition 'für'. It replaces 'für + thing/clause' and helps avoid repetition.
Ich interessiere mich für Politik. -> Ich interessiere mich dafür.
I am interested in politics. -> I am interested in it.
When 'dafür' (for it / in return / on the other hand) stands at the beginning of a main clause (position 1), the verb follows directly in position 2. This change in order is called inversion.
Ich habe viel gearbeitet. Dafür bin ich jetzt sehr müde.
I worked a lot. That's why I'm very tired now.
'Dafür' (for that) is the typical answer to the question 'Wofür?' (What for?). It explains the purpose or function of something.
Wofür brauchst du den Hammer? Dafür, um ein Bild aufzuhängen.
What do you need the hammer for? For hanging up a picture.
In this meaning, 'dafür' (in return) often connects two actions in a kind of 'deal'. The structure is often: 'Du tust X für mich, dafür tue ich Y für dich.' (You do X for me; in return, I do Y for you.)
Du hilfst mir beim Umzug, dafür lade ich dich zum Essen ein.
You help me move; in return, I'll invite you to dinner.
The expression 'nichts dafür können' (to not be able to help it / to not be to blame) is very common and means 'keine Schuld haben' (to have no fault). The person who is innocent is the subject of the sentence.
Ich kann nichts dafür, dass der Zug Verspätung hat.
I can't help it that the train is delayed.
'Dafür' (on the other hand) is often used after a negative statement to introduce a positive, balancing aspect. It frequently appears together with 'aber' (but).
Der Winter ist kalt, aber dafür kann man Ski fahren.
Winter is cold, but on the other hand, you can go skiing.
so that / with it
'damit' and 'dafür' can both be connected to a purpose, but they are used differently. 'damit' can introduce a subordinate clause: it says what goal an action has. As an adverb, it means 'with that' or 'with this thing'. 'dafür' refers to something that something is intended for, or to something given in return. It often stands on its own in the sentence and replaces 'for this thing'. The risk of confusion arises because both can point to a purpose. Remember: 'damit' fits with 'mit etwas' (with something) or with a purpose clause, while 'dafür' fits with 'für etwas' (for something).

Ich habe den kleinen Raum reserviert, weil wir ihn dafür brauchen.
I reserved the small room because we need it for that.
Mein Freund schreibt sich die Fragen auf, damit er sie im Gespräch nicht vergisst.
My friend writes down the questions so that he doesn't forget them during the conversation.