[exporter\Clickhouse] what is the benefit of using toUnixTimestamp64Nano in ORDER BY

[MacNale]

Here the metric table schema created by exporter by default -

(
  ....
 `TimeUnix` DateTime64(9) CODEC(Delta(8), ZSTD(1)),
   ....
)
ENGINE = MergeTree
PARTITION BY toDate(TimeUnix)
ORDER BY (MetricName, Attributes, toUnixTimestamp64Nano(TimeUnix))

git

I am not sure, what is the purpose of toUnixTimestamp64Nano in the ORDER BY. It feels unnecessary. Why not use native type in the ORDER BY as follows.

ORDER BY (MetricName, Attributes, TimeUnix)

Can anybody explain? What is the performance implication?