Welcome to the forum!
The main thing that distinguishes arrangers is that they have a looping autochord function, that allows you to play the background for songs in real time.
In other words, with the autoaccompany feature turned on, you can split the keyboard and use one hand, usually the left, to play the chords to a song, while you can play the melody or solos with the other. You need not sequence the song ahead of time.
There are various styles, or patterns, some factory set, others loaded by the user, that are tailored for certain musical styles or genres.
For example you might pick a style titled 4/4 Rock to play a song such as "Honky Tonk Woman". As you play the chords with the left hand, the bass, rhythm instruments and drums sound in time and in character with the song you are doing. You can vary the tempo as desired and of course when you change chords, all the instruments change with you.
On newer arrangers, there are usually four variations for each of the styles. These allow your "virtual band" to build and change as you progress through the verse, chorus, bridge, etc. of the song. There are also drum and instrument fill-ins that can be triggered either automatically or manually that add variety and realism to the style.
In addition there is a selection of intros and endings for each style, some more elaborate than others.
Regarding your other questions, an arranger can do most anything a workstation can do, including triggering sounds from external sources. Some arrangers have samplers, most have sequencers of varying complexity. Generally speaking, the sequencers in arrangers are not quite as versatile or complete as those in workstations, but this is a generalization.
Other features that most high-end arrangers now have include microphone inputs, vocal harmonizers that allow you to sing harmony with yourself, and multiple digital effect processors.
Many songwriters find that arrangers are very useful in creating song ideas, and indeed complete songs. You need not sequence each individual instrument as you go, but if you come up with something you like you can go back and edit the parts after you save your song as a sequence.
This just scratches the surface. Arranger are widely misunderstood and under-appreciated by the general public and most musicians as well.
Hope this helps. We'll be happy to answer any questions.
DonM
www.donmasonmusic.com