To be ART's trusted friend
I saved a backup copy and hid it where only a trusted friend could find it.
[Network Effect]
When Perihelion found its beloved crew gone after a forced restart, and alien remnant installed on its wormhole drive, it decides to seek help from Muderbot. Not from its university colleagues. When it considered the possibility of itself being deleted by the targets, it made and hid its copy in such a way that only Murderbot could find and reinstall.
This is such an amazing level of trust. And even more amazing when you think about the fact that they had only known each other for just over 3 weeks. Yes, I know that the duration doesn't matter - it's the quality of the time spent together.
When they met, all MB asked for was a ride to RaviHyral. It asked for nothing else, and ART certainly didn't need anything from it. Even though a rogue SecUnit was something new to ART, with its capabilities, it didn't need to speak to it at all. ART could have taken anything under control.
But ART chose to speak to MB. It wanted to get to know it. It decided to befriend it, and help it survive. For MB, it was something extraordinary - to be wanted for something other than its functional values. All ART asked for initially was to watch media with it so that it can understand the context.
ART had no ulterior motive at all to help MB. Knowing this somehow made it easier for MB to ask for ART's help when it faced problems while working for Rami, Tapan and Maro. ART watched MB do much beyond what was required for that contract - which was really just a way to have access to RaviHyral. It saw how far MB went to protect the humans it barely knew. ART, as a research transport, is not supposed to be impulsive or jump to conclusions, but it comes to like and trust MB very, very quickly and completely. Which is heart warming.
Their friendship was founded first by ART really wanting to know this curious rogue SecUnit, mutual, implicit understanding of each other through joint-media viewing, and again by ART's selfless desire to help its new friend survive. It was such a rare, unexpected fortune that MB met ART almost as soon as it started its life as a rogue SecUnit off inventory. Someone who wanted to be its friend, not for its functions.
Everybody else MB met and interacted was through its job (or a pretend job). Fulfilling its functions (esp. the ones it likes, like protecting people, thinking of a clever way to protect them) is very important to Murderbot, certainly. But starting a friendship where nothing except it being itself is asked for must have been very rare and special for it. Just by being itself earned ART's complete trust - with its life, and its crew that it cares about so much.
I'm rambling - sorry. It's just their friendship is so special - to borrow MB's expressions somewhat, I like them being together, and want them to keep on being together!