… because if you just swallow it, it won’t get absorbed. You won’t increase your B12 levels (see graph below).

Some background:
Vitamin B12 Studies in Total Vegetarians (Vegans), Nutrients, 2016

If you’re vegan, it’s a good idea to take a vitamin B12 supplement:

Compared to non-vegetarians, vegetarians have reduced body mass index (BMI), serum cholesterol, serum glucose and blood pressure with a lower mortality rate due to ischemic heart disease (IHD). However, underestimating the correct supplementation of cobalamin (Cbl) [vitamin B12] can nullify these benefits.

We don’t have to take much, in fact, we don’t absorb much from a large dose:

The consumption of oral doses of 1 μg, 10 μg, 50 μg, 500 μg, 1000 μg, are absorbed with an efficiency of 56%, 16%, 3%, 2%, 1.3%, respectively.

In other words, we only absorb:
0.56 micrograms of a 1 mcg dose
1.6 micrograms of a 10 mcg dose
1.5 micrograms of a 50 mcg dose
10 micrograms of a 500 mcg dose
13 micrograms of a 1000 mcg dose

Don’t depend on a multivitamin for vitamin B12:

Using multivitamins can be inefficient and counterproductive for the supplementation of Cbl. The Cbl can be degraded in the presence of vitamin C and copper with the formation of inactive by-products. These compounds can inhibit the transport system interacting with transporter proteins.

The important point: Chew it or let it dissolve in your mouth:

Oral dissolution could be critical in the secretion of the salivary R-binder and its subsequent bond. Since the Cbl would not be dissolved, about 88% could be not absorbed.

Don’t delay:

The sufficiency of bodily stocks should not be a reason to delay supplementation, in light of the fact that the manifestation of the deficiency can occur through often irreversible neurological signs.

Vitamin B12 Studies in Total Vegetarians (Vegans), Journal of Nutritional Medicine, July 2009

For the picture that tells the story:

Look at that! The people who just swallowed their B12? Nothing. Nothing after 6 weeks. NOTHING. OK, a tiny bit. And the same people, when told to chew? Their levels shot right up!

The serum B12 level of seven of 16 adults with a low serum B12 who chewed a 100 μg tablet of B12 once a week for 6 weeks increased by 150%, whereas the serum B12 level increased by only 12% in the nine who gulped down the tablets (with water).
…
The serum B12 increased with oral supplementation only if the tablet could be readily dissolved.

I saw these studies on: