Rootstock: M27 (apple)

Features:

• Size: Extra dwarfing (2m - 2.5m)
• Vigour: Low (not suitable for low vigour scions)
• Precociousness: High (commonly fruits first year after grafting)
• Anchorage: Low (requires permanent staking, more susceptible to drought)
• Resistances: None of note (offers scions no extra protections)
• Size of fruit: Large (often larger than the scion on it's own roots)
• Rate of graft failure: Low (Readily accepts scions, 4-6 weeks to heal)

So, you might take a look at the above and think "Why would I want a tree that's not very vigorous AND has no notable immunity/resistances to pest and disease", and the answer is: for it's precociousness!

Having fruiting scions in only one year can be a major boon if you're looking to test multiple interesting varieties, or even more so, varieties that have not yet fruited (aka new/seedling varieties!).
Getting a seedling to flower in the in less than 3-5 years is basically impossible, but the extremely high levels of precociousness afforded by M27 can speed things up significantly. Seedlings grafted to M27 have been known to flower as early their first year on the rootstock (!), though more commonly in second and third year.

To me, someone looking to breed their own new varieties, that would save years of waiting to be able to test new seedlings.

But for most people, looking to graft a known scion, the part that's most important would probably be the dwarfing nature, full size/large fruit. Perfect for backyard orchards/potted orchards, but may need more care when it comes to pest and disease, depending on your area.