Rundpinnen brukes som strikkepinne.

Because rundpinnen is the definite singular form: the circular needle.

In Norwegian, the definite article is usually attached to the end of the noun instead of being a separate word in front.

• en rundpinne = a circular needle
• rundpinnen = the circular needle

So the -en ending works like English the.

Norwegian normally puts definiteness on the noun itself.

So instead of saying something like English the circular needle, Norwegian often says:

• rundpinnen

That final -en already means the.
This is very common in Norwegian:

• en bil = a car
• bilen = the car

Because brukes is a passive form.

• bruker = uses / is using
• brukes = is used

So:

• Rundpinnen brukes ... = The circular needle is used ...

This is a very common Norwegian way to make the passive:
verb + -s

Examples:

• selger = sells
• selges = is sold
• å bruke = to use
• brukes = is used

They are very close in meaning, yes.

• Rundpinnen brukes som strikkepinne.
• Rundpinnen blir brukt som strikkepinne.

Both can mean The circular needle is used as a knitting needle.

The -s passive (brukes) is often a bit more compact and neutral.
The blir + past participle form (blir brukt) is also very common and can sometimes feel a little more explicit.

In a simple sentence like this, both are natural.

Here som means as.

So:

• brukes som strikkepinne = is used as a knitting needle

This use of som is very common when something functions in a certain role:

• Han jobber som lærer. = He works as a teacher.
• Dette brukes som verktøy. = This is used as a tool.

After som, Norwegian often leaves out the article when talking about role, function, or category.

So som strikkepinne means as a knitting needle in a general functional sense.

Compare:

• Hun jobber som lærer. = She works as a teacher.
• Det brukes som våpen. = It is used as a weapon.

You can sometimes see an article after som, but without the article is very common when describing what something functions as.

Strikkepinne is the more general word for a knitting needle.

Rundpinne is a more specific type: a circular knitting needle.

So the sentence is basically saying that the circular needle is being used in the role of a knitting needle. In other words, a rundpinne is a kind of strikkepinne, but the two words are not equally specific.

Also, both are compound nouns:

• rund + pinne = circular + needle
• strikke + pinne = knit + needle

It is singular.

• rundpinne = a circular needle
• rundpinnen = the circular needle

A plural form would be different, for example:

• rundpinner = circular needles
• rundpinnene = the circular needles

So in your sentence, it is talking about one circular needle.

In Norwegian, the present tense verb is the same for all persons and numbers. That makes verb conjugation simpler than in English.

For å bruke in the present tense:

• jeg bruker
• du bruker
• han/hun bruker
• vi bruker
• de bruker

And in the passive:

• brukes stays brukes

So Norwegian does not do something like English I use / he uses.

Yes, but the basic sentence you have is the normal straightforward order:

• Rundpinnen brukes som strikkepinne.

That is subject + verb + complement.

You could move things around for emphasis in a larger sentence, but then Norwegian follows the usual verb-second rule. For example:

• Som strikkepinne brukes rundpinnen sjelden.

That kind of rearrangement is possible, but much less basic. For learners, the original order is the one to remember first.

Then you would use the indefinite noun:

• En rundpinne brukes som strikkepinne.

That means A circular needle is used as a knitting needle or A circular needle is used for knitting-needle purposes, depending on context.

So compare:

• Rundpinnen brukes ... = The circular needle is used ...
• En rundpinne brukes ... = A circular needle is used ...

The difference is the same basic definite/indefinite difference as the vs a in English.