The reason being it would retain ARM's neutrality where an Apple or Intel purchase might close off competing ecosystems on the Android, Windows, or Apple side which ever be the case.
With IBM ownership ARM would now have the benefit of world class fabs on par with Intel along with IBM's pre-existing willingness to share process tech with ARM partners.
Not to mention a massive addition to R&D budget and resources.
The benefits to IBM would be obvious, it would be the semi player in the post-pc space after being upstaged from PC's by Wintel 30 years ago. Additionally, it could help ARM advance it's server efforts and give IBM a low power server line to partner Power.