Nested code - blocks of code inside blocks of code - is eventually necessary, but increases complexity. This is why keeping the code as flat as
possible, by avoiding unnecessary nesting, is considered a good practice.
Merging if statements when possible will decrease the nesting of the code and improve its readability.
Code like
if (condition1)
{
    if (condition2)           // Noncompliant
    {
        // ...
    }
}
Will be more readable as
if (condition1 && condition2) // Compliant
{
    // ...
}