It means "because." However, unlike "because," which is a subordinating conjunction, "for" in this sense of meaning "because," is a coordinating conjunction.

English has only seven coordinating conjunctions (i.e., and, for, but, yet, so, or, nor) but many, many subordinating conjunctions. While a subordinating conjunction (e.g., because) introduces a subordinate clause, a type of dependent clause, meaning it depends on an independent clause and cannot stand on its own as a sentence, when a coordinating conjunction (e.g., "for") introduces a caluse, it is a coordinate clause, a type of independent clause, it can be conjoined with another independent clause, but because it's a type of independent clause, it can also stand on its own as a sentence.

Therefore, though "for" in definition means "because" at the start of the Tolkien quotes you cited above, since "for" in function is a coordinating conjunction, the use of "for" to start the Tolkien quotes you cited means that each is able to grammatically stand on its own as a sentence, unlike if Tolkien had started them with "because" instead.

It means "because." However, unlike "because," which is a subordinating conjunction, "for" in this sense of meaning "because," is a coordinating conjunction.

English has only seven coordinating conjunctions (i.e., and, for, but, yet, so, or, nor) but many, many subordinating conjunctions. While a subordinating conjunction (e.g., because) introduces a subordinate clause, a type of dependent clause, meaning it depends on an independent clause and cannot stand on its own as a sentence, when a coordinating conjunction (e.g., "for") introduces a clause, it is a coordinate clause, a type of independent clause that can be conjoined with another independent clause via that coordinating conjunction, but since it's a type of independent clause, it can also stand on its own as a sentence.

Therefore, though "for" in definition means "because" at the start of the Tolkien quotes you cited above, since "for" in function is a coordinating conjunction, the use of "for" to start the Tolkien quotes you cited means that each is able to grammatically stand on its own as a sentence, unlike if Tolkien had started them with "because" instead.

It means "because." However, unlike "because," which is a subordinating conjunction, "for" in this sense of meaning "because," is a coordinating conjunction. While a subordinating conjunction, like "because," introduces a subordinate clause, a type of dependent clause, meaning it depends on an independent clause and cannot stand on its own as a sentence, a coordinating conjunction, like "for," introduces a coordinate clause, a type of independent clause, so it can stand on its own as a sentence.

Therefore, while "for" means "because" in the Tolkien quotes you cited, since "for" is a coordinating conjunction, they introduce clauses that are able to stand on their own as sentences, unlike if Tolkien had started those sentences with "because" instead.

It means "because." However, unlike "because," which is a subordinating conjunction, "for" in this sense of meaning "because," is a coordinating conjunction.

English has only seven coordinating conjunctions (i.e., and, for, but, yet, so, or, nor) but many, many subordinating conjunctions. While a subordinating conjunction (e.g., because) introduces a subordinate clause, a type of dependent clause, meaning it depends on an independent clause and cannot stand on its own as a sentence, when a coordinating conjunction (e.g., "for") introduces a caluse, it is a coordinate clause, a type of independent clause, it can be conjoined with another independent clause, but because it's a type of independent clause, it can also stand on its own as a sentence.

Therefore, though "for" in definition means "because" at the start of the Tolkien quotes you cited above, since "for" in function is a coordinating conjunction, the use of "for" to start the Tolkien quotes you cited means that each is able to grammatically stand on its own as a sentence, unlike if Tolkien had started them with "because" instead.