I am guessing two reasons.  quick & easy 

```bash
$ time fallocate -l 1G test.img

real    0m0.007s
user    0m0.002s
sys     0m0.000s

$ time dd if=/dev/zero of=test.img count=1024000 bs=1024
1024000+0 records in
1024000+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 2.37254 s, 442 MB/s

real    0m2.373s
user    0m0.443s
sys     0m1.881s

```