The units of an investment fund are subject to fluctuations in value and their prices vary accordingly.
With your fund savings plan, you regularly invest a constant amount in the fund of your choice. The advantage for you: you benefit from the cost-average effect. With the same amount, you buy lots of fund units when prices are low and few when prices are high.
This helps you to keep your investments within your budget. With payments that are always the same, you compensate for fluctuations in the value of fund units over the long term and avoid the problem of investing "at the wrong time".