Management fees fund the operating costs of a venture firm: salaries, rent, travel, due diligence. The standard structure is 2% per year of committed capital during the 'investment period' (typically 5 years), stepping down after to ~1.5% on invested capital. Over a fund's life, total fees often consume 12 to 18% of committed capital.
Management fees are paid regardless of fund performance, which means GPs get paid even if returns are zero. Carry exists to align GPs with performance; management fees exist to keep the lights on.