I don't know all of the details but here are the basics:
It's used to pay fees on the platform. SLT is a unit of account and allows for voting on platform rules. There are SLT holding requirements.. I think its 1% of your STO investment amount which gets market bought automatically unless you already have enough to cover it. They have mentioned discounted fees or maybe some other benefit to holding more than required. Also holding requirement to issue an ABT ( i dont know the amount). They've mentioned it may get additional utility when they add banking and credit card in the future (later this year according to one of their posts). If the platform continues to be successful I think it could become pretty valuable over time.