Why use fixtures instead of global variables when testing?

I'm new to testing, please bear with me. If I'm lacking fundamental conceptual knowledge, let me know in a comment. I'd like to improve this question (or realize it shouldn't be asked), but I can't tell what's wrong from downvotes alone.

Question

When using pytest, is there a downside to using global variables instead of fixtures?

Code (with a global variable)

DATASET = FlickrPortraitMaskDataset(root=ROOT)

class TestFlickrPortraitMaskDataset(object):

    def test_len(self):
        nb_portraits = len(get_fnames(ROOT + "portraits/"))
        assert len(DATASET) == nb_portraits

    def test_getitem(self):
        idx = random.randint(0, len(DATASET) - 1)
        sample = DATASET[idx]
        assert isinstance(sample, dict)

Output

TestFlickrPortraitMaskDataset::test_len PASSED
TestFlickrPortraitMaskDataset::test_getitem PASSED

Code (with a fixture)

@pytest.fixture(scope="module")
def dataset():
    return FlickrPortraitMaskDataset(root=ROOT)

class TestFlickrPortraitMaskDataset(object):

    def test_len(self, dataset):
        nb_portraits = len(get_fnames(ROOT + "portraits/"))
        assert len(dataset) == nb_portraits

    def test_getitem(self, dataset):
        idx = random.randint(0, len(dataset) - 1)
        sample = dataset[idx]
        assert isinstance(sample, dict)

Output

TestFlickrPortraitMaskDataset::test_len PASSED
TestFlickrPortraitMaskDataset::test_getitem PASSED

The output is the same.

---

Common

import random

import pytest

from portraitseg.utils import get_fnames
from portraitseg.pytorch_datasets import FlickrPortraitMaskDataset


ROOT = "../data/portraits/flickr/cropped/"