Precision data conversion.
Decimal (Base 10) uses 1,000 as the multiplier and is the standard for storage manufacturers (HDD/SSD). Binary (Base 2) uses 1,024 and is how operating systems like Windows calculate file sizes. This is why a "1TB" drive often shows as ~931GB in your system settings.
In the standard decimal system (SI), there are 1,000 Megabytes (MB) in 1 Gigabyte (GB). In the binary system (often referred to as Gibibytes), there are 1,024 Megabytes in 1 Gigabyte.
Hardware companies sell drives using decimal math ($1\text{GB} = 10^9$ bytes). Computers read that same drive using binary math ($1\text{GB} = 2^{30}$ bytes). The "missing" space is simply a difference in calculation methods.
Petabytes are used for massive scale data, such as cloud storage infrastructure, server farms, and big data analytics. 1 Petabyte is equivalent to 1,000 Terabytes (Decimal) or 1,024 Terabytes (Binary).
A "bit" (lower case 'b') is the smallest unit of data, while a "Byte" (upper case 'B') consists of 8 bits. Usually, internet speeds are measured in bits (Mbps), while file sizes and storage are measured in Bytes (MB/GB).