It depends what the end goal is.
If the goal is to play in ‘isolation’, like you would an acoustic guitar round the campfire, and not record or process your sound in any way what so ever – then a portable amp type thing works.
If you want to connect your synth to bigger, better sounding speakers, be able to route the sound to a computer, add (virtual) effects to your synth and ultimately record your music – then you’ll need an audio interface. Broadly speaking.
A great beginner audio interface is the Focusrite Scarlett 2i2. It has two outputs to your active monitor speakers, and two inputs for your synths. It goes to USB to your PC.
Personally, I would never listen to a synth dry. Some people like raw synth sounds, but I only enjoy subtractive synths with some EQ and a reverb/delay. I don’t have hardware racks, so I used to connect my BassStation 2 to my interface and apply FX inside Ableton. I could record that into my tracks that way. My ‘studio’ (read: lounge) monitors reproduced the sound.
At the end of next week I’ll have a Focusrite 2i2 for sale. If anyones interested let me know. There are plenty on the Bay though.