Often they are brand new tweeters. Often, all I get is a blank header with their twitter name and a one word description - possibly writer. Sometime not even that.
The single most important role of a writer is to communicate but that's the one thing they have failed to do. I have no idea who they are or what they are writing.
If you're going to set up a new social network page, be it twitter, facebook or instagram, you need to show the world who you are and what you do.
Use a picture that represents what you do.
Choose it carefully - it's how the world will see you - and make sure you use a picture you have a licence for. If you can't afford to buy one from one of the many sites offering the kind of pictures you need, you could join Unsplash, where generous photographers offer images for free, or use a photograph snapped on your phone. Flowers, your garden , your cat, dog, gerbil, where you live - or the region you write about. An image that says something about you. Just make sure you take it landscape style, because that's the shape of those headers.
Or you can use the space to show how, or where, you write - do you use a notebook and pen, a laptop? Work in a cafe, the library, at the kitchen table, or in the garden?
And tell me what you're writing. Contemporary or historical romance? Women's fiction.?Crime, erotica, fantasy?
Say if, like me, you're a member of RWA in America, or the Romantic Novelists' Association in the UK. I always follow the RNA's NWS members but I won't if all I see is a blank header and "@xxxxx hasn't tweeted yet".
That first tweet should be to introduce yourself, entertain, inform. Make me want to follow you. Show me what interests you. Tell me about your research. If you're a writer it will be good practice - because there will come a time when you'll want me to read your book. If I enjoyed your tweets, the chances are that I'll enjoy your book.