Writing the DNA sequence is done with nuclosides that have three phosphates attached to them, and two of these phosphate are thrown away back into "phosphate pool" of cell. What are main reasons triphosphate structure is used? Conformational changes in the polymerase seems like it could be one of the primary ones.
Questions of "why" in biology are not completely proper - evolution simply happened that way, and the reasons we suggest are rationalizations that may not encompass the "real" reason. Still...
Enzymatic reactions are inherently reversible. If you make an enzyme that converts XMPs (nucleoside monophosphates) into DNA, then the same enzyme can convert DNA to XMPs also. Plus, there's "entropy" to consider, which is to say, if you pop off an XMP it goes flying off into the solution, it's a lot harder to round it back up again. Last but not least -- we're talking about DNA here. Every time DNA is degraded, even if the other strand is still intact, there is some chance that some mutation will occur with harmful effect.
All this can be set right by wasting a lot of energy every time DNA is extended. We snip off two phosphates - a pyrophosphate which is rapidly broken down by pyrophosphatase. The way equilibrium works, that second step pulls the reaction forward because it makes sure there is no pyrophosphate left lying around. Even if a freak surge of thermal energy at the level of molecular vibration would allow the enzyme to reattach it, the pyrophosphate no longer exists.
As OP suggested, was correct. From Stryer Biochemistry, https://www.ncbi.nlm.nih.gov/books/NBK22374/
"the binding of a nucleoside triphosphate (NTP) to DNA polymerase induces a conformational change, generating a tight pocket for the base pair consisting of the NTP and its partner on the template strand. "
Further, as Mike Serfas answer states, "Enzymatic reactions are inherently reversible. If you make an enzyme that converts XMPs (nucleoside monophosphates) into DNA, then the same enzyme can convert DNA to XMPs also. " This was also a good anwer to the question, since the OP had not considered that. The OP associates is with how the substrate for translation from RNA to proteins in ribosome is not amino acids but tRNA + amino acids.
Then, as some angry comments from some random person mentioned, and the good answer from Mike Serfas also mentioned, and, the standard explanation (but, not what OP was fishing for, OP was looking more in-depth) always focuses around, entropy etc.
There was nothing wrong with the question, no formally legitimate reason to close it. It helped the learning process of the OP, and potentially others that acesss this collaboratively generated content on this public Q&A site.