A forex position produces its result in the counter-currency of the pair, not in whatever currency your account holds. If those differ, the broker converts. Most firms offer a choice of USD, EUR, GBP and a handful of others at sign-up, and changing it later usually means opening a new account.
The choice matters in three places. It sets the currency your pip-value is finally expressed in, it decides whether a conversion-fee applies on every closed trade, and it makes your equity curve partly a bet on your own home currency if you spend in something else.
Traders who live outside the dollar zone often still pick USD because most pairs have a dollar leg, which removes one conversion. Others pick their spending currency so the balance means something in real life.
Example: a USD account trades 1 standard-lot of EUR/GBP. One pip is GBP 10. With GBP/USD at 1.2700 that pip is worth 10 x 1.2700 = $12.70, and a 40-pip winner books $508 rather than a round number.
Related: conversion-fee, counter-currency, pip-value, pip-value-cross-pair