An artist's portfolio is an edited collection of their best artwork intended to showcase an artist's style or method of work. A portfolio is used by artists to show their versatility with different examples of current work.
No two artists will ever work the same. Some may seem to carry similar styles, but upon closer inspection they'll differ in the same way a signature differs from person to person. An artists personal interests will also affect this. They're going to put more heart into something that really holds their interest and for each artist this will also be different.
When you're in the market for a new tattoo, a lot of thought is put into the design that would be perfect for you. You should put in just as much time researching the artist who will do the piece. No matter what, their style will influence the way your piece turns out. So if you're looking for something that's more on the realistic side, an artist who's style is based around traditional Americana might not be the best choice, because the two are very different. One relies on shading with very little hard lines and the other is very heavy on linework and dark shading. Some artists do a bit of everything, but don't just assume that. Not all artists are the same and this is not McDonald's.
If you're going around from studio to studio searching for an artist, the first thing you should do is look through the artists portfolios. All of them. In every studio you walk into. If you're not doing that, you're short changing yourself.
I can't tell you how many people wander into the studio where I work, hold up a picture on a phone and say "how much?". We advise them to look through portfolios, but no... they just want that magical price for something they found online and they're out the door to see if they can find a cheaper price. That's just silly. This is forever. And as much as you try to tell them that, only a few will actually listen.
We're artists. We draw too. Pinterest and Google are just going to show you other people's tattoos and drawings that a few million other people already have. Don't be someone else. We're not interested in turning you into someone else. We're far more interested in turning you into the best, most original version of yourself. Don't you want that too? If you sit down and consult with the artist, they might be able to guide you in a direction with your design that you hadn't considered before.
If you give ten different artists a rose to draw, you're going to wind up with ten very different roses. Some might take more time than others because some styles and techniques make that necessary. Some might be realistic. Some might be old school. Some might be new school or neo-traditional, tribal, etc... it really depends on that artist. They will vary vastly. But how would you know how that rose would turn out unless you actually take a look at what that artist can do for you? That's part of what the portfolio is for. Usually, if the artist has a strong point or a lean in a certain direction as far as style or subject matter, that's what they put in their portfolio.
So really, the last thing they want to hear is "I love the look of the work you do, give me this random design drawn by someone else that isn't even close to what you're capable of..."