If the write speed is equal to or lower than the read speed, why do we need to use a buffer?

Even if the write speed is equal to or lower than the read speed, a buffer is useful for smoothing out temporary peaks and increasing I/O efficiency. The buffer allows accumulating data and performing batch operations, which reduces the number of system calls and context switches. Additionally, the buffer helps avoid blocking and delays, providing asynchrony between the producer and consumer of data.

Example: if writing occurs evenly, but reading may have short delays, the buffer will store data and allow reading to operate without stalls.