A useful dress is one you enjoy wearing, not one that merely looks right in a photograph. The shape, fabric and length should suit both the event and the way you move through it.
Let the occasion shape the dress
For an ordinary day, a washable dress you can walk and sit in may be ideal. At work, a simple dress can be styled with a jacket or cardigan according to the dress code. For a celebration, you may want a more distinctive fabric or detail, while still allowing enough comfort for the whole event.
Try the garment, not just the size
Sizes vary between labels. Check how the dress fits at the shoulders, waist and hips, and whether the lining or seams feel comfortable. Sit down, reach forward and walk in it. Consider whether you will need an alteration and whether that cost makes sense.
Check the cost beyond the price tag
Before deciding, ask whether the dress needs alterations, special cleaning or a layer you do not already own. A sale price can lose its appeal if the garment spends most of its life waiting for the one event it fits. Take a photograph in natural light and revisit it the next day if you have time.
Name two places you would realistically wear the dress again. A familiar jacket may make it suitable for work; a different layer might take it to a family gathering. If you cannot think of a second outing, that is useful information, not a rule against buying it. Our accessories guide can help you restyle what you own. For a broader approach to choosing lasting pieces, read the women's wardrobe guide.