It would be useful in code like this:

func (c *conn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
	level := sql.IsolationLevel(opts.Isolation)
	switch level {
	case sql.LevelDefault:
		// …
	case sql.LevelReadUncommitted:
		// …
	default:
		return nil, fmt.Errorf("isolation level %s is not supported", level)
	}

Without it, all driver authors have to implement the mapping in the driver itself, or not bother with helpful error messages. Examples:

• mysql – does not work as expected due to string([int]) does not work as author probably thought it does
• lib/pq – numerical level is returned, not very helpful
• pgx – numerical level is returned, not very helpful