When people discuss slide guitar, they often presume the guitar will be open-tuned to a chord. This is fair enough as it’s a time-honoured method that allows a lot of flexibility.
However, unless you’re going to play the whole gig in an open tuning, it might feel a bit excessive to keep retuning or to carry an extra guitar for a couple of songs. If you’re just looking to play solos, or even just getting into slide, it isn’t necessary to go to such lengths.