You're actually describing a technique called scratching the side of the beaker to induce crystallization, and it's not always necessary, but it can be helpful in certain situations. Here's why:

1. Providing nucleation sites:

* Crystallization is a process where dissolved molecules come together to form a solid lattice structure. This process requires a *nucleation site* – a small solid particle or surface imperfection that provides a starting point for the crystal structure to form.

* Scratching the beaker creates these imperfections. Tiny scratches on the glass surface provide a place for molecules to start clustering and forming crystals.

2. Overcoming supersaturation:

* A solution is considered supersaturated when it holds more dissolved solute than it normally could at a given temperature. This is a very unstable state.

* Scratching can help "tip the scales" and initiate crystallization. The small scratches provide the necessary nucleation sites for the excess solute to start coming out of solution and forming crystals.

When is it necessary?

* Solutions that are very close to saturation: If a solution is only slightly supersaturated, it may take a long time for crystals to form on their own. Scratching can speed up the process.

* Solutions that are "stubborn": Some compounds are more prone to supersaturation and may not readily crystallize. Scratching can help jumpstart the process.

Important Notes:

* Not always necessary: In some cases, crystals will form readily without any assistance.

* Other methods exist: Other methods of inducing crystallization include seeding (adding a small crystal of the desired compound) and adding a seed crystal of the desired material.

* Gentle scratching is key: Avoid scratching the beaker too hard, as this could cause the glass to break or introduce impurities into the solution.

Let me know if you'd like more details on any of these aspects!