Although the glomerulus transfers many of the substances from the blood to the tubule, other transfers of substances from the blood vessels (the peritubular capillaries) to the tubule take place further along to regulate the pH level (acidity/alkalinity) in the blood.
Here is a simple overview of secretion and absorption in the nephron.
In the glomerulus, substances are filtered out of the blood. The peritubular capillaries reabsorb substances that the body wants to retain. At the same time, the peritubular capillaries secrete substances into the tubule to regulate pH level.
Do you ever hear tuba players practise? No, they always do it in secret (tubular secretion).