Genome indexing with hisat2 and samtools

Hello,
I found this code in Biostar handbook (RNA-Seq by Example) in section about genome indexing.

# The reference genome.
IDX=refs/genome.fa

# Build the genome index.
hisat2-build $IDX $IDX

# Index the reference genome with samtools.
samtools faidx $IDX

I was wondering about samtools indexing step. I’ve never seen anyone using it before. Isn’t hisat2-build not enough to properly index genome?