Original image, 1200 x 900 px
Resized image, 1080 x 1080 px
Without cropping and white borders (they're transparent, download the image to see they really are)
Many tools will try to resize your image with cropping applied without preserving the aspect ratio. Thus, the picture doesn't show what it meant in the first place.
Even if such resizer tools save the aspect ratio, there is another problem. You will see white borders because a resizer tool saved the aspect ratio. Hence it left spare space. There are two options for filling it: with white color or with transparency.
When they choose white color, you'll see white borders, usually on the top and at the bottom. However, when the images should have the exact size(in social networks, for example), they look not so good(why are white areas there, right?).
This online photo resizer makes those areas transparent. Thus, you won't see white borders on your images when you resize them. So, it saves the aspect ratio(hence, there is no cropping) and doesn't add a white background(borders).