What a lot of fans did not take away from watching "That Which Survives" was that it was about an internal conflict waged within the Kalandan Outpost computer. At the very end of the episode, Kirk & Spock realize, after viewing Losira's video log, that Losira had transferred a copy of her mental & emotional essence into the base computer. In the Captain's own words, "The computer was too perfect. It copied so much of Losira's personality into the replica that it felt guilt, regret, at killing, and that bought us the time we needed to destroy it."
In other words, the Kalandan Cube Computer was schizo.
Losira's personality was replicated so thoroughly into the computer that it began to clash with itself. What we see then is Losira's caring, benevolent nature vs. the cold, brutal logic of a machine programmed to defend.
Why was the Enterprise catapulted 990.7 light-years away instead being simply destroyed? Losira won that round, at the expense of Ensign Wyatt, the transporter operator, who the computer was able to force Losira to kill off.
The computer succeeds in forcing Losira into killing Watkins in engineering & geologist D'Amato as he searched for water. She almost kills Sulu with her deadly touch & very nearly destroyed Kirk & the landing party by overloading a phaser to explode.
With the ship speeding back to the Kalandan Outpost, Losira buys time for the Enterprise by sabotaging the engines to exceed its maximum velocity. In doing so, she wins a crucial round against the computer by giving the Enterprise the time it needed to save itself while making it look as though the Enterprise was doomed to fly apart from extreme speed. The computer would have just transported the ship into the insides of a sun if it had its way. With the Enterprise engines going wild but still fixable, it allowed the ship to get back just in time to rescue the landing party from the 3 Losira holograms who were created in answer to the landing party's game of ring-around Losira.
As for why use such a bizzare method of defense, namely Losira's "I Am For You, I Want To Touch You." Death Touch, I see her as the ultimate smart bomb. Each Losira hologram is only lethal towards 1 specific individual. How much more precise of an attack could you possibly get? No collateral damage to worry about. The hologram also profiles like a suicide bomber, being able to actively scout, improvise, & adapt to a dynamic situation. It also doesn't hurt that it's indestructible, as Sulu found out when his phaser blasts had no effect on Losira. Losira's physical beauty also didn't hinder its ability to distract 'D'Amato from calling for help either.
For Losira, her beauty survives in every way, and that's why it's my favorite TOS episode.