What is the point of "9–10 The target chooses its behavior"?  If the creature failed a saving throw, why does it get another chance to avoid the spell's effects?

it choses it's behavior among the other option's limiting it but giving it just a sip of choice

It still can't take Bonus Actions or Reactions.

they should have given polymorph to the trickery domain instead of this, and non- detection is worse than this.. legacy class had a better spell list

Could it be interpreted as choosing any of the other three effects?

My interpretation is that, on a 9 or 10, the affected creature (or character) chooses its behavior (limited to Actions only, no BA nor Reactions), but is still under the effect of the spell. Thus, their next turn they have to roll the d10 again, maybe sending them walking off a cliff.

An example: my character has just cast Confusion of a pair of Goblins.    Let's see what they do....

• Goblin Able failed the original WIS save and rolls the d10. They get a 5 = Goblin A stands there looking stinky and ugly.
  • To conclude their turn, they roll the WIS save: Failed again!
• Goblin Bob also failed the original WIS save and rolls the d10. They get a 10! Goblin B chooses to use their crossbow to fire at me. Attack roll = 18 (Ouch), DamageRoll = 6 (OUCH!!).
  • To conclude their turn, they roll the WIS save: Failed again!
• My turn: I clench my teeth as I pull the crossbow bolt from my leg, tossing it aside; then, I consume a Healing Potion (ahh, that feels better).
• Goblin Able must roll the d10. They get a 7 (sweet!). Goblin A uses one of his two scimitars to slash at Goblin B. Attack roll = 17; damage roll = 3 (I smile). Goblin A cannot use it's Bonus Action to use the 2nd scimitar because it doesn't have the dual wielding Feat.
  • To conclude their turn, they roll the WIS Save: 19 = Success!
    • On their next turn(s) Goblin A is no longer under the effect of the Confusion Spell; they don't have to roll the d10
• Goblin Bob must roll the d10. The get a 1 (bwaa-haa-haa!). They roll a d4 and use all their movement to move ... 2=East ... bumping into the locked gate they're supposed to be guarding.
• My Turn: With my high Passive Perception, I recognize that Goblin A has shaken off the effects of the Confusion Spell, so I target them, of course, with my Lightning Bolt. They fail the DEX save and their charred corpse collapses to the dungeon floor.
• Goblin Able ... oh, yeah, they're dead... next!
• Goblin Bob must roll the d10. They get another 10 (lucky them). They fire their crossbow at me, but miss.
  • To conclude their turn, they roll the WIS save ... (wow, this one is very ... um ... not wise) they get a 5.
    • On their next turn they will again have to roll the d10 to see what they get to do.
• My turn: I turn to face the discussion forum and take the monologue action:

"So, you can see, my friends,

the lucky (but not so wise) Bob was able to attack me,

but remained under the thumb of that accursed d10.

Every now and then, they might act out their own greatest fantasy

(attacking me);

however, at other times,

they turn on their heel and march 30ft,

short-bow in hand,

straight toward my buddy,

the Raging, Dragon-born Barbarian with his +2 Maul!"