Das Sparschwein dient als Trinkgelddose.

Sparschwein is a noun, and German nouns have grammatical gender. Here it is das Sparschwein because the last part of the compound is Schwein, and das Schwein is neuter.

A useful rule:

• In German compound nouns, the last element usually determines the gender.
• So:
  • das Schwein → das Sparschwein
  • die Dose → die Trinkgelddose

Dient is the third-person singular present tense form of the verb dienen.

So:

• dienen = to serve
• es dient = it serves

In this sentence, Das Sparschwein dient ... means The piggy bank serves ... or is used ...

Yes, but specifically in the sense of having a function or being used for a purpose.

So dienen here means:

• to serve as
• to function as
• to be used as

It does not mean serving food or working as a waiter.

Example:

• Dieses Werkzeug dient zum Öffnen der Dose. = This tool serves to open the can.
• Das Sparschwein dient als Trinkgelddose. = The piggy bank serves as a tip jar.

Als is used to show a role, function, or identity.

So dient als Trinkgelddose means:

• serves as a tip jar
• functions as a tip jar

A very common pattern is:

etwas dient als + noun

Examples:

• Der Karton dient als Tisch. = The box serves as a table.
• Die Flasche dient als Vase. = The bottle serves as a vase.

Because als and wie do different jobs.

• als = as, showing a role or function
• wie = like, showing comparison

So:

• Das Sparschwein dient als Trinkgelddose. = The piggy bank serves as a tip jar.
• Das Sparschwein sieht wie eine Trinkgelddose aus. = The piggy bank looks like a tip jar.

In your sentence, the piggy bank is actually being used in that role, so als is the correct choice.

After als, German often leaves out the article when talking about someone’s or something’s role or function.

So:

• als Lehrer = as a teacher
• als Beispiel = as an example
• als Trinkgelddose = as a tip jar

Using no article sounds natural because the sentence is about function, not about one specific jar.

If you added an article, it would usually sound less idiomatic here.

It is a compound noun, which is very common in German.

It is made of:

• Trinkgeld = tip
• Dose = container / tin / jar / box, depending on context

So Trinkgelddose literally means something like:

• tip container
• tip jar
• tip box

The last part, Dose, determines the gender:

• die Dose
• therefore die Trinkgelddose

Because German usually writes compound nouns as one single word.

English often uses separate words:

• piggy bank
• tip jar

German usually combines them:

• Sparschwein
• Trinkgelddose

This is completely normal in German, even when the word gets long.

You could, but it changes the nuance.

• Das Sparschwein ist eine Trinkgelddose.
  This sounds more like you are saying it is a tip jar by identity.

• Das Sparschwein dient als Trinkgelddose.
  This emphasizes that it is being used as a tip jar.

So dient als is better if you want to focus on purpose or function.

It is the normal German word for piggy bank.

Literally, it is:

• sparen = to save
• Schwein = pig

So it is a savings pig, which matches the English idea of a piggy bank.

Because this is a normal German main clause, and German main clauses usually follow the verb-second rule.

The structure is:

• Das Sparschwein = first element
• dient = finite verb in second position
• als Trinkgelddose = the rest

This is standard German word order.

Yes. In German, all nouns are capitalized.

So in this sentence:

• Sparschwein is capitalized because it is a noun
• Trinkgelddose is capitalized because it is a noun

But:

• das is not capitalized here because it is an article in the middle of a sentence
• dient is not capitalized because it is a verb

This is one of the most noticeable spelling rules in German.