There are two reasons to invest rather than save. One is inflation. The other is compound growth.
A savings account might earn 4-5% right now, but that is unusually high. Over long periods, savings accounts have historically earned less than inflation. Investing in real businesses and assets has historically beaten inflation significantly.
Yusuf puts £200/month into a savings account earning 2%. After 30 years: £98,000. Ahmed puts £200/month into a halal index fund returning 7% per year. After 30 years: £243,000. Same monthly amount, 145,000 difference. The gap comes from compound growth.
Accumulating wealth is not contrary to Islamic values — provided it is done ethically. Islamic teaching encourages enterprise and the productive use of resources. Zakat (the annual charitable contribution on wealth) is only paid by those who have wealth above the nisab threshold, implying that building wealth is expected and encouraged.